DEC, 


OF 
Y 


1909 


WASHINGTON, 
PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON 





B 
VOLNEY M. SPALDING 


DISTRIBUTION AND MOVEMENTS 
DESERT PLANTS 





CARNEGIE INSTITUTION OF WASHINGTON 


PUBLICATION No. 113 


PRESS OF GIBSON BROS. 
WASHINGTON, D. C. 


CONTENTS. 
Page 
OTE COIES, «Gt 0 SN cc rr A eae ee eee I~4 
DEEP eASOOCIATIONS AND HABITATS, .-...'..-.03 0-4 o0sees0ss0 03s we 5-2 
(1) The river and irrigating ditches; association of hygrophytes........ : 
(2) The river banks; association of cottonwoods and willows........... 8 
(3) The flood-plain; mesquite forest association...................0.. 9 
(4) Salt spots; association of salt-bushes...................... ae 13 
(5) The wash; palo verde-catclaw association. ...............000.005. 14 
(lee eMNeSisIKG SlOPeS 25.5...) 2 ans +s euers Ee Oe Ee ee. 16 
(Ce reocote- bush associa tion, £5.08 ot a. oe ee ee 16 
(Gye raticeria association sagcctek « tlre edge pees eee, eee gee 17 
Mie LGN A sah cas. ial, tease, ot icant its eit pee. EET. tick Pam? eae SE ae 18 
(a) Association of Fouguieria and Parkinsonia microphylla....... 18 
(b) Association of Cereus giganteus and Encelia farinosa......... 18 
KC wi Opi ASSOCIA i@tbin chute r Sale a ee ant ene ti ee 19 
(GeV PUN IONANG ASSOCIATION wate ch tee atte his lease 19 
(5) ‘Superficial soil dayerss association of annuals 5 «21040 0 a ass 20 
(a)\eonade-loving cpectessiy. oo. nik. Aiea. Sieg ees A Bas, Beet ad 20 
COL Isnt lovin ne sertes ey a A det ka iy ne ce eee es 21 
(Oe aia ctlic ati svt DINE PEAILS snes 9. ora dubctite «pd cot eite Aes ae 22 
Pichens or the Desert :Labottory domatt, 046 wwe hie ass dabeyan 24 
Oe Miscella nents initoduced SNCCIES., 7 cos couse eke ee as uNdee 27 
LIMA COCAL MI MSTRIBUTIONIONROPRCINST, Anmiew a¥ ea ds 5 SHEE. fh od. es 29-66 
Hops piic na isoll relations, fe 2 ee eos dee. Pel 29 
ORCL UPENOSU LED, Set PREP tyr ee ates ee A OE RE, 29 
aE TALE HLG Ma ee Be ae dinss AM Che thin 38 tei Aaa Bea dyin @ 3PM t 30 
FEU POCA TILTED IMac xi hon Re Mes OMe yh te Rew teen 8 2 eo ed Gate 31 
PCL OTIC PAIN rhe es... AE a Oe meee sc ee EP Ba 
EA OLD TO DOU Aine x OM Bat Sie eee Nant aera aie Suny’ Sylpeive ep ayers hs 36 
COMPARA Ca SUUCIOS Terai s PIT oe wise jetn ou, eine Dee wales baa Te a 
NEedS a idles PENCIES Ol GISDEL S31. We aes 2) ee Peg el. Ge OI es 47 
Tea 10 ie COM Perition aN SUCCESSION: fy occ pee. oe bao sin osha 2 51 
Habussnd stticturesirelatedto, distribution. f.-0: 6.03 Bi a Ly 
a he. root-systemol Cereus c1vanteus, 6443 6 ese fda ed ae vss 59 
TAD ENVIRONMENTAL AND? HISTORICAL, FACTORS)... 0. foe ee eee cee ene 67-102 
The geology of the vicinity of the Tumamoe Hills............5......... 67 
eta (abl yam en et eh te a dda ped y atek.k 73 
Cy er eR tt ee ree tN aN No Sines wip. ee v2 Sy she 74 
tee i ern re ee oie a oan ss Sie Sip TE Ge aye 79 
Piletemlstomtesescrt | abOratory COmainy vec 8 bak feel ns ne es be ess 83 
ETE OS COW ATA EN TRTTES fo) oe Boe PM lt a Oe As iad a a a 94 
(ISSR ES ONSEN lho © Dec Renata ia aie! ok! a nr 94 
Pemipers etre tecorde ea ianen ee cu avs et 5 Fhe steko avoss 96 
DiCastitemenia OmevanGla MOU RE Raa Pee re eo kk Saw elk 99 
PITIAES Ol 1 ett Ota OLY COLA ie ees in Se ee a 101 
IV. VEGETATION GROUPS OF THE DESERT LABORATORY DOMAIN.......... 103-112 
Ve. LH ORIGINGORS DESERT CHLORAS ace aia al tts Aes kee hep aes 2 113-119 
Rote uy Wd NIE LITSCUSSION tac 4 ee ene, In nae 9.2 8d wiring lee I 21-137 
‘Lhe plantassociat ions, 24r Greree eam fulton 1. telat es Wa SS. 121 
Heaphiorelationsy. 2005.21, 22S Pee CAO IG Are aC sy. B22 
Concurrent action! different factors oq. 0 es cee as, Phen | 127 
OCA UMD Y CINETILS oo ha <tae ieaen e Tie oens  e 9S: oo COD va" 
General movementss.ic0 SOe es ee Oe Gere et eta., Plas 134 
RRR UMERERE Vere ee et is fee oe ee a ee it OM eet ye nae batt ai 139-142 
SEES SI 9 La 3 cn ele ae Ke Par eee Soy ie eae nn ae 143-144 
1 


813418 


LIST OFePi ares, 


BEATE Lai feel ce is wives AST Wi wis Se eet at oe 0 eR ane tel eee ce 
The Tumamoc Hills from the South. Tumamoc Hill, on which the Desert 
Laboratory stands, on the left, connected by a ridge with Sentinel Hill 

on the right. Santa Catalina Mountains in background. 

East side of Tumamoc Hill looking North; flood-plain of Santa Cruz River 
at right. In the foreground a heavy growth of Encelia farinosa, with 
other characteristic vegetation. 

PUGATE 2 od 4S eee hates oi hele soho ee atid gi nya ee eee Re gta ee eRe ee a 
Salt River at a point near Phoenix, Arizona, soon after high water. 

West side of Tumamoc Hill, looking West across the wash and adjacent 
slopes. Tucson Mountains in background. The path leads up the 
Tucson slope. 

15) OW Oke, ae ene Panam Pe OPS ERTS PoC ook ne yeh, ere ae MERC OES 
Cut bank of Santa Cruz River near Tucson, showing results of erosion. 

Roots of mesquite exposed. 

Dry bed of Santa Cruz River. Willows and cottonwoods on banks. 

PYUATE (Aso cited oo ais ot ce Sin a ne ee ee 
Edge of mesquite forest, on plain of Santa Cruz River, South of Tucson. 
Mesquite on sand-dune, Salton Basin. 

PUA TE 95 Or desis aralatae 8 sae a Ne nh os ole REGS GARE ee EE, ve ee 
Salt-spot in Salton Basin. Clumps of Allenrolfia near the center and masses 

of Atriplex nearer the margin, as seen at the left. This area, now cov- 
ered by water, was photographed before the last invasion of the Salton 
Sea. 

‘The wash Northwest of Desert Laboratory. Lining its banks are Cercrdiwm 

torreyanum, Acacia greggi, A. constricta, Prosopis velutina, and Celtis 


pallida. 
PATE “6 05 dag cice San oss 4 wep whe im 905, wo, Reidy 05) 9 fahaoigeee SRR 
Single individual of Cercidium torreyanum. Branching opuntias in back- 
ground. 
Ephedra trifurca in wash West of Desert Laboratory. 
fA 90. lg a ree Soe Cem MR ce oe. AR 


Creosote-bush a year old, showing comparative extent of top and root-system. 

Vegetation of the wash. In foreground Yucca elata. 

PLATE «BOs Som @Som vn os eon Mors an EDM ae oe ie pPONS ede nae te ota ee eS 

Zone of creosote-bush at the foot of Tumamoc Hill on the North. In the 
background Santa Catalina Mountains beyond the flood-plain of the 
Santa Cruz River. 

Tucson slope beyond the wash West of Desert Laboratory. I vanseria del- 
tordea and Opuntia fulgida constitute the characteristic vegetation at 
this point. 

PLATE: 6 ogc acu 4c ie 6h 0 la OG ok ea oop cine ee ee 

Slope of Tumamoc Hill showing a strong growth of characteristic vegetation 
in which Fouquieria splendens and Parkinsonia microphylla are con- 
spicuous. 

Right side of gulch near Laboratory, with generally south exposure. 

PLATE: 10. i505 Saisie, Hedges wi Sis eating oe ere ee ee ee ee 

Left side of gulch near Laboratory, with generally north. exposure. 

Lippia wright forming a nearly pure growth on the left side of the gulch, 
near its head. 

PLATE OUT aiysg atk. ayes aps. bs 5 nie. Sn ps satel ean ect ee es et 

Mesquite infested with mistletoe (Phoradendron calijornicum). 

Rank growth of winter annuals, at this place chiefly Amsinckia spectabilis. 

PLATE 12. Lichens on North face of rocks above Desert Laboratory.......... 


IV 


Facing 
Page. 


IO 


14 


14 


16 


16 


18 


Pies 


26 


LIST OF PLATES. 


PLATE 13. Map of Desert Laboratory domain, showing distribution of Encelia 
PORE DIGD Gea ee RN tise 4.5 xa he gas ss pia pry oi atin we eee oe 
PLATE 14. Map of Desert Laboratory domain, showing distribution of Larrea 
tridentai® Over an area Of dbout 1,800 acres. 2... 6. eek ee enews 
PLATE 15. Map showing distribution of sahuaro (giant cactus) on the Tumamoc 
Pana eeieiiie ye CGO, ATIZ0NA .. . 0c. cc 4k ahaa beep Wh ee chee 
PLATE 16. Map of West part of Laboratory domain with land adjacent on the 
North and South, showing physiographic and soil areas and distribution 
of Cereus giganteus. Area 700 acres. May, 1907. ‘Total number of 
Poetical MlaOts Ol SIAR CACt~AS, SIF. i. ene Oe ee nes Bai a 
PLATE 17. Map showing distribution of Cercidium torreyanum on West part of 
Laboratory domain and vicinity, Tucson, Arizona. November, 1907. 
Rea OC GEG i ET ae tend aS ete ne Sl ee a SG At cack 9 phe ee tes ee 
PLATE 18. Desert Laboratory domain and vicinity, showing approximate distri- 
bution of Prosopis velutina over an area of about 1,800 acres 
PLATE 19. Soil map of a part of Solomonsville sheet, Arizolia «eee en 
PLATE 20. Desert Laboratory domain and vicinity, showing i invasion of Ei enue 
cicutartum. ‘The numbered patches show the position of areas occu- 
pied by alfilaria, in February, 1906 
ae eee eee ere eran ce tea ik  e AUR GAS Ueaapetain ade DSP Ve She aceal mar een ove 
Permanently marked area No. 7, showing invasion of Erodiwm cicutarium. 
In center of area a large individual of Parkinsonia microphylla. 
Anchoring roots of a sahuaro measuring 6.8 meters high and 39 cm. in diam 
eter at the base. 
PLATE 22. Desert Laboratory domain and vicinity, showing invasion of Hordeum 
murinum. ‘The numbered patches show the position of areas occupied 
PONS AI eT QOL Wi preteen Pmt ole cena wip needa gk cinee nce aioe ttt akan acinar de 
PLATE 23. Curve showing expansion and contraction of sahuaro (Cereus gigan- 
teus) from March 3, 1906, to May 26, 1907. Precipitation is indicated 
jay ean Ser vA Fans Cte babe yn an este, Buran eeoee, PA aes ea eee es a we 
PLATE 24. Chart of the root-system of sahuaro (solid lines), creosote-bush 
(jotted lines), and Pariinsonta (broken lines). .... 20. ase ive oe ee 
ED ees eG eee i Ms Schad 2 vlata sh orate oe Wales le doe, fae eign bl « Weauan Y wae 
Cucopa Mountains at western margin of Colorado Delta. 
Volcanic hills in Great Basin, near Hazen, Nevada. 
Typical slope West end of Santa Catalina Mountains from the North. 
PLATE 26. Reservation and vicinity of the Desert Botanical Laboratory of Tuc- 
fon MAtizone ourveyed OCCtOber, TQ00 nt. ae suede be es ke toe 
PLATE 27. Geological map Tumamoc Hills and vicinity Tucson, Arizona...... 
Prateerss Crves.0);sol-miorsttire, | winamoc Hilla.) i. neces wen eae 
iPUATE 208 Curves of soll-moisture, Larrea Slope... 0. ea ce eee ey es 
PLATE 30. Curves of soil-moisture, WLS) Od ec ade tee OM oe Bah tea eae 
PrAate 31. Curves of soil-moisture, Santa Cruz plain. ..- 26. 6124s, os or eee es 


Cle, ee 6 le. a) fe) vl 6, B16 6) We 6 Ose wl ce) a) ie OL Whe) we Oak Cele im 


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in 2022 with funding from 
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it 


Distribution and Movements of Desert 
: Plants. 


INTRODUCTION. 


In the study which has led to the present paper it was deemed best to 
establish at the outset certain definite limits within which more exact and 
prolonged observational and experimental work could be undertaken, 
and beyond which, as opportunity offered, comparative studies could 
from time to time be made. Naturally the domain of the Desert Lab- 
oratory was chosen for the limited area within which the greater part of 
the work has been done, and the following pages are, first of all, a report 
of this work. The broader relations, which involve comparative studies 
in other regions and under different conditions are, as yet, owing to insuf- 
ficient data, less clearly established; but they have received such atten- 
tion as it has thus far been possible to give. 

Tumamoc Hill (plate 1), on the northern slope of which the Desert 
Botanical Laboratory of the Carnegie Institution of Washington is situ- 
ated, is an outlying peak close to the eastern slope of the Tucson Range, 
which latter extends from the neighborhood of the old mission of San 
Xavier del Bac, south of Tucson, in a general northwesterly direction 
upwards of 25 miles, parallel to the valley of the Santa Cruz River. Its 
geographical position and topographical features are alike favorable for 
an investigation of factors by which the distribution and present asso- 
ciations of plants here represented have been determined. 

Standing, as it does, on the border-land between the plateau region of 
central and eastern Arizona, on the one hand, and the great desert plain 
that stretches westward to the Colorado River and its delta, on the other; 
connected, moreover, by the valley of the Santa Cruz and by broken 
ranges of mountains and hills with the highlands of northern Mexico, 
there mingle within the narrow limits of this one hill and the adjacent 
valleys many plants represented in the widely different floras of Cali- 
fornia, New Mexico, and Texas; and along the north and south pathway 
in which it stands may be noted the northern limit of various Sonoran 
species, as they drop out one by one. Its altitude, slightly upwards of 
3,000 feet, gives at this latitude, in connection with its topographical 
features and variety of soils, an ensemble of conditions under which plants 


of very different habits occur together; well-marked mesophytes, for 
1 


2 DISTRIBUTION AND MOVEMEN'’S OF DESERT PLAN‘S. 


example, growing luxuriantly in close proximity to the most pronounced 
types of xerophytes. Thus, as regards historic, edaphic, and climatic 
factors, the tract selected offers exceptional advantages for such a study 
as is here undertaken. 

The fact that Tumamoc Hill owes its origin to volcanic agencies, inter- 
mittently active from the Tertiary to within Pleistocene history, fixes the 
period within which its flora has become established. During the whole 
of this period, if we accept the views of present-day geologists, generally 
arid conditions have prevailed throughout this region, except at higher 
altitudes. So far, then, as available evidence goes, the desert plants now 
growing here originated in their present places or came to them as desert 
plants; there is no evidence that they became such after their arrival; 
and taking them as we find them we are under the necessity of empha- 
sizing the efficiency of existing agencies as factors in their present actual 
distribution, without invoking other causes to explain phenomena trace- 
able to those now in operation. 

It is a matter of congratulation that the existing flora has been so little 
disturbed or modified by human agency. ‘The few introduced weeds, 
though in some cases conspicuous, have made little real impression upon 
its character, and there is no evidence that extermination of species has 
followed the occupation of Tumamoc Hill by the prehistoric people, the 
outlines of whose dwellings are still distinctly traced upon its summit, 
nor even through the ruthless work of modern quarrymen, by which its 
sides are here and there marred. Changes due to human agency have 
. undoubtedly occurred, and are seen most plainly along the various trails 
and wagon-roads that have been constructed; but there is no reason to 
suppose that thus far they are of more than the most superficial character. 
Certain changes are beginning to follow the fencing of the reservation, 
coincidently with the exclusion of hunters and cattle; but the flora in all 
its essential features, as it now exists, is presented to us as the final phase, 
thus far, of the natural movements and adjustments that have been 
taking place, broadly speaking, since early Pleistocene times. 

It can hardly be doubted that in this region, during the period from 
the Tertiary to the present, two distinct floras of different origin have 
existed side by side as they do to-day; the one including the desert plants 
of the arid plains and lower elevations, the other the mesophytes of the 
mountains. As just stated, the species belonging to the former have 
apparently originated where they now live, or at least have undergone 
migrations of very limited extent compared with those which many of 
the latter have made as representatives of plants which in earlier days 
migrated southward from arctic regions, and which still, at the present 
time, exhibit features of the old Eurasian stock. ‘There is, of course, 
more or less intermingling of the elements of these very diverse floras, 
where climatic and soil conditions make this possible, but one has only 


SPALDING 


PLATE 1 


i 
ree 
ge) 





The Tumamoc Hills from the south. Tumamoc Hill, on which the Desert Laboratory 
stands, on the left, connected by a ridge with Sentinel Hill on the nght. Santa Catalina 
Mountains in background. 


DSTI AE 





East side of Tumamoc Hill looking north; flood-plain of Santa Cruz River at right. In the 
foreground a heavy growth of Encelia farinosa, with other charactenistic vegetation. 


CAMPBELL ART CO., ELIZABETH, N. J. 





INTRODUCTION. 3 


to place side by side the plants of Tumamoc Hill and those of Mount 
Lemmon, a few miles away, to realize how essentially different in their 
fundamental characteristics the two floras are. 

In dealing with the former, it is found that with it are closely asso- 
ciated, though in more or less distinct contrast, the plants of the neigh- 
boring valleys, and it has seemed necessary, therefore, to include with 
the Laboratory domain the natural setting of Tumamoc Hill, that is, the 
ground immediately adjacent, extending from the bed of the Santa Cruz 
River on the east to the boundary of the Laboratory reservation on the 
west, and from the flood-plain of the river on the north to the same topo- 
graphic horizon on the south, in this way delimiting the area studied by 
natural rather than artificial boundaries. Thus bounded, this area includes 
approximately 4 square miles and exhibits all of the characteristic plant 
associations within the immediate neighborhood of the Desert Laboratory. 

As already indicated, attention will be given in this paper chiefly to 
problems of distribution as related to causes now in operation. The 
physical factors which modify and more or less control the associations 
of plants are under investigation by different members of the staff of the 
Desert Laboratory, but for the most part can not as yet be formulated 
quantitatively. At this stage of progress it is a distinct advantage to 
be able to make use of the more exact knowledge in certain departments 
which, by the courtesy of my colleagues and other specialists, is given in 
the accompanying contributions. The section on the origin of desert 
floras has been written by Dr. D. T. MacDougal; that on geological his- 
tory by Prof. C. F. Tolman; and that on soils by Dr. B. E. Livingston. 
The list of species, and their arrangement in vegetation groups, has been 
_ prepared by Prof. J. J. Thornber, who has otherwise rendered valuable 
aid. Dr. W. A. Cannon has generously placed at my disposal, in advance 
of publication elsewhere, his observations on the root topography of 
certain species. Prof. Bruce Fink has contributed an account of the 
lichens; and the maps of local distribution, with accompanying notes, 
have been prepared by Mr. J. C. Blumer. 

Through this cooperation it is fair to say that there is perhaps no simi- 
lar area that has been more thoroughly studied. Yet it must be frankly 
admitted that, at the present time, interpretation of the data thus far 
gained is only possible in part. It would be nothing but the baldest 
pretense, for example, to tabulate observations of light intensity or tem- 
perature, even though, as with the latter, they have been laboriously 
continued through many months, and to attempt to correlate these accu- 
rately with observed facts of distribution; nevertheless, it is precisely 
by the multiplication of such data that we hope for the ultimate attain- 
ment of results that may be expressed with mathematical exactness. 
The justification of publishing so far in advance of the attainment of this 
goal lies in part in the necessity of preserving vanishing data and in part 


4 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


in the necessity of accumulating and recording as extended a series of 
comparative observations as possible to serve as a basis for generalization. 

I wish to express my thanks to Prof. B. L. Robinson, of Harvard Uni- © 
versity; Messrs. F. V. Coville, of the U. 5. Department of Agriculture, 
and J. N. Rose, of the National Herbarium; Prof. H. M. Hall and Mr. 
T. S. Brandegee, of the University of California; and Prof. J. J. Thornber, 
of the University of Arizona, for free access to the herbaria under their 
charge, and for generous personal assistance at such times as it has been 
my privilege to consult them. I take pleasure also in acknowledging 
my indebtedness to Mr. S. B. Parish, of San Bernardino, who has kindly 
placed at my disposal numerous facts of local distribution, and to others, 
especially my colleagues of the Desert Laboratory, who in various ways 
have rendered substantial aid in the course of the work. 


CHAPTER I. 
PLANT ASSOCIATIONS AND HABITATS. 





For reasons already stated, the plant associations and habitats of 
Tumamoce Hill and the adjacent valley have been made the basis of the 
studies of desert plants reported in the following pages. ‘The topography 
of the tract with which we are immediately concerned is so intimately 
related to its plant life that it becomes necessary at the outset to refer 
to its salient features. These are (a) the Santa Cruz River and its flood- 
plain, (b) the long mesa-like slopes leading from this to the mountains, 
and (c) the Tumamoc Hills. Proceeding westward from the city of 
Tucson, we come first to the river, which forms the eastern boundary 
of the area studied. Beyond this, the flat flood-plain, at this point a 
mile or less in width, stretches to the edge of the slope, from which a 
little beyond rises the group of three hills to which has been given collec- 
tively the name of the highest one, Tumamoe, which attains a height of 
742 feet above the flood-plain. ; 

The cliffs and steep slopes of Tumamoc Hill are in turn succeeded on 
the west by the mesa-like slopes through which ‘‘the wash,” with its 
numerous branches, passes on its course to the flood-plain north of the 
Laboratory. The mountains farther to the west, seen in plate 2, belong 
to the Tucson Range, and are beyond the tract now under consideration. 
By observing the more striking features of the landscape the various 
habitats may be partly located. Further details are given in connection 
with the maps of distribution. 

It will be seen that within this area, with a radius of hardly more than 
a mile, there are habitats exceedingly diverse in character, inhabited by 
plants correspondingly widely different, and that these differences, more- 
over, are distinctly edaphic. It is evident at once that the water-relation, 
first of all as regards soil-water, is a determining factor, through which, 
under essentially identical atmospheric conditions, plants of the most 
widely different habits and requirements are growing almost side by side. 
The same thing is seen the world over in both temperate and tropical 
zones, but it is especially striking here from the close juxtaposition of 
extreme ecological types. 

The general features of the vegetation of Tumamoc Hill have been 
made familiar through papers of Lloyd (1904), MacDougal (1908), and 
others that have appeared within the past few years. Accordingly it is 
only necessary here to refer to them in the briefest manner by way of 
preparation for the study of plant associations which follows, and the 


analysis of the flora given in a later section. 
. 5 


6 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


The plants of the hill and the slopes adjacent are essentially xerophytic, 
including cacti, the creosote-bush, the ocotillo, and other conspicuous and 
well-known desert forms. Many of the plants of the valley just below, 
on the other hand, are distinctively mesophytic. Such are the willows 
and cottonwoods of the river banks, the ash and elder near irrigating 
ditches, and various introduced species. Even on the hill there are some 
species growing in shady places that neither in habit nor structure show 
any special adaptation to desert conditions. Thus within the area selected 
for study there are great differences of biological types. 

With the exception of the trees already named and a very few addi- 
tional ones, the woody plants are mostly shrubs, and these exhibit great 
differences of stature and habit. Many of these are low, as if stunted, 
but a few, such as Fouquterta, for example, make a vigorous growth, as 
if growing under ideal conditions, and become thereby conspicuous fea- 
tures of the landscape. Collectively they exhibit in perfection the well- 
known characters of desert shrubs, some with greatly reduced leaves, 
others with spines, thickened epidermis, hairy coverings, fleshy reservoirs 
of water, and still other familiar devices by which protection against 
animals and against excessive transpiration has been secured. There are 
two or three climbers and a few vegetable parasites, of which Phoradendron 
and Orthocarpus are the most obtrusive, the latter, at the season of flower- 
ing, covering the hillsides with wide patches of color. 

The most striking effects, however, are produced in favorable seasons, 
when after sufficient rainfall the winter annuals appear in early spring 
-in countless multitudes, some with fine green leaves covering the ground 
like grass, others, such as the California poppy, producing masses of color 
that may be seen for miles away, and still others, less conspicuous, that 
by their rapid growth and local abundance produce a pleasing and almost 
bewildering variety where a few days before the oppressive monotony 
of brown earth and bare rocks prevailed. A similar, though hardly as 
impressive, change occurs after the summer rains, when another and 
very different set of annuals bloom and form their seeds. 

On the north faces of the rocks there are lichens of the crustaceous 
form, a few scattering mosses occur, and even a species of liverwort is 
found after the winter rains. A number of parasitic forms, chiefly of the 
Uredineze and Perisporiacee, occur on herbaceous hosts, and a very few 
species of green and blue-green alge are found in irrigating ditches and on 
moist ground; but, on the whole, cryptogamic plants constitute an incon- 
spicuous and relatively unimportant part of the vegetation and the flora. 
Grasses are fairly well represented, and although here, as elsewhere in 
tropical and subtropical regions, they grow in tufts and form no true 
sod, yet some of them, such as Hilaria, so far approach this as to form 
collectively broad patches visible at a distance, and become successful 
competitors for areas that, but for their presence, would be occupied by 
other plants. 


SPALDING 


West side of Tumamoc Hill looking west across the wash and adjacent slopes. 


Mountains in background. The path leads up the Tucson slope. 


CAMPBELL ART CO., ELIZABETH, N, J. 


PLATE 2 





Tucson 


lie Liviast 
OF ist 
GRIVERSITY GF iLLtkGiS 





PLANT ASSOCIATIONS AND HABITATS. 7 


The various elements of the flora that have been mentioned are grouped 
in societies, or associations, evidently related to different combinations 
of physical factors, such as water-supply and temperature, and also to 
historical factors which have been concerned in their establishment in 
their present habitats. In the following pages, which of necessity are 
largely descriptive, the part played by these several factors is the ultimate 
object of inquiry. 


(1) THE RIVER AND IRRIGATING DITCHES; ASSOCIATION OF HYGROPHYTES. 


The Santa Cruz River has the general character of streams in the south- 
western United States. For months at a time its bed is empty, but at 
the period of summer and winter rains it is not infrequently filled with a 
raging torrent, which has measured no less than 4,012 cubic feet of water 
per second at the Tucson bridge, flowing at the rate of 9 feet per second.! 
When thus swollen by rain it is a turbid, impetuous stream, bearing along 
branches of trees and débris of various kinds to be deposited at different 
points as the water lowers. At these times of flood its power of erosion 
is very great, and in a few hours the banks in places are deeply cut away, 
acres of fertile land being swept into the devouring current. 

In the course of a few days a small and harmless stream, a few inches 
in depth, is quietly flowing in its channel, and later, after the rains are 
over, even this disappears, leaving the bed of the river, as far as appears 
on the surface, only a collection of variously assorted and mingled masses 
of sand and gravel. Beneath the surface, however, is still at some depth 
a body of water, and from this, even in the driest times, the cottonwoods 
and willows along its banks draw an abundant supply. (Plate 3; com- 
pare plate 2, the Salt River after high water.) 

Irom what has just been stated, it is evident that the conditions are 
in general unfavorable for the growth of aquatic vegetation. The rapid- 
ity o1 the current when the river is “up, the turbidity of the stream, 
and the long period when no water appears above ground, are not condu- 
cive to the normal development of water-plants. Yet there are places 
where these do secure a foothold and grow with remarkable vigor. Espe- 
cially is this true of the irrigating ditches, in which green alge frequently 
accumulate in such quantities that they have to be cleaned out to prevent 
the channels from becoming choked. Great masses of Cladophora are 
often thrown out on the banks for long distances. Hydrodictyon is also 
abundant. Potamogeton pustilus is frequently conspicuous, and in pools 
of the river channel, where the force of the current is lessened, the water- 
cress (Radicula nasturtium-aquaticum) grows in luxuriant masses.? 





1Measurements by Prof. G. E. P. Smith, of the Arizona Experiment Station. The 
date of this flow was January 17, 1907. ‘The next highest flood measured was 3,200 
cubic feet per second on November 28, 1905. 

Synonyms and authorities for plant names used in this paper are given in the sec- 
tion on vegetation groups by Prof. J. J. Thornber. In his list plant names recently 
changed by systematists are given, as far as possible, in their latest accepted form. 


8 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


(2) THE RIVER BANKS; ASSOCIATION OF COTTONWOODS AND WILLOWS. 


The banks of the river are lined with willows (Salix nigra) and cotton- 
woods (Populus fremontiw), which constitute their conspicuous vegeta- 
tion, while the arrow-weed (Pluchea sericea) is of frequent occurrence. 
Another plant conspicuously present where the banks are sandy is Aster 
spinosus, which also extends out on the flood-plain to some distance. 
As in similar situations elsewhere, a considerable number of annual plants 
succeed in gaining a foothold on the shifting sands of the river, but none 
of these are distinctively characteristic of this special habitat. It is the 
cottonwoods and willows that constitute the typical vegetation of the 
river-banks. It may be noted in passing that the willow of the Santa 
Cruz River does not appear to be specifically distinct from Salix nigra, 
which ranges from New Brunswick to Florida and westward to California. 
The cottonwood (Populus fremonti), according to Bray (1904), appar- 
ently takes the place of Populus deltovdes west of the one-hundredth 
meridian, extending to western California and into Lower California and 
northern Mexico. 

The flora of the river and its banks is seen to be meager as compared 
with those of regions with abundant rainfall. In the Huron River, at 
Ann Arbor, Michigan—a fair example for the eastern United States 
there are over a dozen species of Potamogeton as against the single species 
of the Santa Cruz River at Tucson, and while along the banks of the 
Huron River, at the place named, there are 10 or more species of willows, 
we have on the banks of the Santa Cruz, at this point, barely one. It 
is evident that the total of conditions here is unfavorable to the great 
majority of species of similar habitats in humid regions, and with the 
admirable arrangements for dispersal which these plants possess, it is not 
unlikely that various species have been brought here time and again that 
have thus far failed to gain a permanent foothold. It is quite probable, 
however, that various species not found here would thrive if they could 
once get a start. Seeds of willows, for example, may often have reached 
the river when there was not sufficient moisture in its bed to result in 
their germination, and even when started they would still find conditions 
far less favorable for survival than along the banks of streams in regions 
of abundant rainfall and more humid atmosphere. 

In its physical features, then, and in its flora (and fauna as well) the 
Santa Cruz is a characteristic river of the region through which its course 
runs. However luxuriant may be the growth of the few species that 
have established themselves along its banks, and however its waters may 
locally be choked at times with dense masses of one or more aquatic spe- 
cies, yet as regards both physiognomy and life it is essentially a river of 
the arid Southwest, presenting the same general characters as those of 
the Salt River, the Gila and others of this region, and no botanist could 





SPALDING PLATE 3 





Cut bank of Santa Cruz River near Tucson, showing results of erosion. 
Roots of mesquite exposed. 


“ep HD 





Dry bed of Santa Cruz River. Willows and cottonwoods on banks. 


CAMPBELL ART CO., ELIZABETH, N, J, 


THE LIBeARY 


GF THE 


beivensitY OP wuaois 


- 





PLANT ASSOCIATIONS AND HABITATS. ! 


for a moment fail to recognize this fact, especially as just beyond its 
banks there is growing on every hand the mesquite, the everywhere- 
present species of the Lower Sonoran zone. 

Thus even an abundant water-supply, with the strong growth of hygro- 
phytes determined by it, altogether fails to reproduce here, except in the 
most superficial way, the plant associations of rivers in the eastern United 
States. There may be an approach to a mesophytic forest, where willows 
and cottonwoods grow thick along the banks of the Santa Cruz, but not 
a mesophytic forest of the East or North. Other factors than water- 
supply, however potent this may be, are to be reckoned with in attempt- 


ing to account for the wide differences of plant and animal life that are 
here observed. 


(3) THE FLOOD-PLAIN; MESQUITE FOREST ASSOCIATION. 


The flood-plain of the Santa Cruz River is essentially the same in its 
physical characteristics as those of other rivers of the Southwest. A 
deep, fine alluvium, closely resembling the Maricopa sandy loam of the 
Gila and Salt River Valleys, the product of a long period of erosion and 
deposition, fills the valley from the river-bank to the mesa-like slopes at 
the foot of the mountains. It approaches adobe in texture, and bricks 
of a rather inferior quality are manufactured from it, but it is of a high 
degree of fertility, as evidenced by the crops of grain, fruits, and vege- 
tables which it produces. 

As already stated, the flood-plain in the immediate vicinity of the 
river suffers from erosion when the stream is high. Plate 3 shows the 
condition of affairs at a point less than a mile south of Tucson, where 
the roots of mesquite and other plants have been exposed along the deeply 
cut channel. 

According to statements of residents, this extensive erosion is of recent 
date. Previous to the advent of cattlemen some 20 years ago, and the 
destructive effects of over-pasturing, the valley of the Santa Cruz had a 
luxuriant growth of saccaton and other vegetation, which prevented 
the cutting of cha1nels, and the water spread out over the whole valley 
instead of flowing through the deep cuts it has since made; tules grew 
thickly in the springy places, and a fine forest of mesquite covered the 
ground.’ At present the effects of such erosion are seen most plainly 
from the point below Tucson already indicated to one about 2 miles 
above the city. It is inevitable that such changes, where they occur, 
should be followed by a lowering of the water-table of the flood-plain, 





1] have given the commonly received version of the cause of the cutting of oe 
nels of the Santa Cruz, but have since been told by Mr. Herbert Brown, of the see 
Post, that about 20 years ago, certain old settlers undertook to poe water” a 
a point about 2 miles down the river, where there were springs, and in e ex £0 ae 
plish this most easily cut a channel for a little distance, expecting the river w = 
it rose to do the rest. “Their expectations were fully realized, for the river scoure 


out the cut and kept on with its work, as already indicated. 


10 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


resulting in more or less pronounced changes of vegetation. Neverthe- 
less, although the formerly abundant saccaton has disappeared, and 
along with it doubtless other species, the vegetation of the flood-plain 
shows clearly enough what were in earlier times, and are still, its essential 
features. 

The dominant species is the mesquite (Prosopis), here in the form 
velutina, a highly characteristic species of the Lower Sonoran zone as 
defined by Merriam, but extending in its various forms far beyond.’ It 
is by preference a plant of low flats, though it occurs far beyond these 
on the uplands, in situations where a sufficient water-supply is obtainable. 
In the neighborhood of Tucson the mesquite ranges in size from a mere 
shrub a few feet high to a tree 2 feet or more in diameter and upwards 
of 4o feet in height. Such trees grow thickly on the bottom-land near 
the old mission of San Xavier, forming the fine forest that stretches for 
miles up the river, in the shade of which grows a rank vegetation similar 
to that of eastern mesophytic forests in luxuriance. 

The habits of the mesquite are popularly well known, and its presence 
is taken to indicate a good water-supply. Its roots extend widely, to a 
distance of 50 or 60 feet according to credible observers, and possibly to 
as great depth; and when cut green its wood tissue, which is hard and 
heavy, carries a large percentage of water, precisely as do the hardwoods 
of the eastern United States, and strikingly different from the creosote- 
bush, its near neighbor on the slopes beyond the flood-plain. At the 
same time, the general structural peculiarities of the mesquite are xero- 
phytic. It is commonly armed with spines, and its coriaceous leaves 
are well protected against excessive transpiration. Itisa plant requiring 
a better supply of water than many of its associates, yet well adapted to 
the low relative humidity of the desert air, and its occurrence beyond its 
own special area, ranging as it does to the top of Tumamoc Hill, in spots 
where a soil retentive of moisture affords the conditions it needs, corre- 
sponds with this peculiarity. Thus it is, in a sense, a desert plant, yet 
one of high water requirement—characteristics which it shares with 
various other species. 

The capacity of this plant for taking possession of wide areas beyond 
its earlier limits of distribution is of special interest. According to 
Bray (1904), “its spread northward and eastward from the Rio Grande 
country during the past 50 years has been a marked phenomenon. By 
its invasion, mile after mile of treeless plain and prairie have been won 
and reduced to a characteristic orchard-like landscape. It has traveled 
northward over the Staked Plains, covering half their area, and has 





‘IT have made no attempt to separate or delimit by geographical boundaries the 
forms of the mesquite which, under the names of Prosopis glandulosa, with its center 
in Texas, P. velutena in Arizona, and P. dulcis in northern Mexico and beyond, repre- 
sent the views of systematists concerning the closely related species or varieties of 
this interesting plant as it occurs in the southwestern United States and adjacent 
parts of Mexico. 


SPALDING PLATE 4 





Edge of mesquite forest, on plain of Santa Cruz River south of Tucson. 


2 


ead 
A Re 





Mesquite on sand-dune, Salton Basin. 


CAMPBELL ART CO., ELIZABETH, N. J. 





PLANT ASSOCIATIONS AND HABITATS. iL 


passed over Oklahoma and into southwest Kansas. This encroachment 
of mesquite is partially accounted for by its weed-like capacity for occu- 
pying new ground . . . and by the influence of cattle in scattering 
the beans.’’ Such extensive and rapid invasion as is here described has 
not been observed in Arizona, so far as the writer is aware, but the same 
characteristics are noted here—its preference for low, flat areas, with 
compact soils, but with a marked capacity for extending beyond these 
on to higher ground, its successful reproduction where it is really at home, 
and the tenacity with which it holds its place where once established. 
Its endurance of conditions to which various other species are less fitted 
is well seen in many places from Texas to California, and particularly 
in the Salton Basin, where great mounds of sand are blown about the 
mesquite, which keeps on growing, its branches extending beyond the 
rising heap or dune, until the latter has reached a height of several meters, 
the protruding branches still covering it with an apparently healthy and 
normal growth. » 

All in all, in the success with which its relatively rapid dissemination 
is accomplished, in its ready adjustment to widely varying conditions, 
its utilization of available water, and in other ways, this plant is to be 
considered one of the best adapted of our desert species, and its wide 
distribution and present successful invasion of new areas mark it, not as 
a decadent species, but as one in pristine vigor, from which apparently 
several others are in process of evolution. 

Associated with the mesquite are a number of characteristic species, 
some of which are confined to the flood-plain, while others extend with 
it beyond these limits. Of the latter Acacza constricta is a conspicuous 
representative. Its habits are essentially those of the mesquite as to 
water requirements, and it closely resembles this species in its xerophytic 
structure. The two grow side by side near the river and to the summit 
of Tumamoc Hill, in precisely the same situations, one being, apparently, 
the ecological equivalent of the other. Other species behave differently. 
Acacia greggw grows with the mesquite in its lower range, but not on the 
hill above, and the same is true of Condalia lycioides. Both of these 
exhibit distinctively xerophytic structures, but both are as yet adapted 
to a somewhat more restricted range of soil conditions than are the mes- 
quite and Acacia constricta. Sambucus mexicana and Fraxinus velutina, 
also of this association, are still more limited in their range, growing near 
irrigating ditches, hardly affecting even the edge of the mesa-like slopes, 
and structurally are to be thought of as essentially mesophytic. 

Passing through the list of plants belonging to this association, it is 
seen that the same relations are maintained—certain species are strictly 
confined to the flood-plain, while others occur rather widely beyond it; 
it is to be noted, however, that even those that range most freely exhibit 
their best development on the flood-plain and not on the hillsides. The 


12 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


mesquite, which attains the size of a tree on the plain, is a mere shrub on 
the hill, and Bigelowra hartwegu, which grows with the luxuriant habits 
of a weed in the former situation, is scattering and small in the latter. 

It appears, then, that the flood-plain is the natural habitat of a number 
of species, many of which are incapable of successful growth elsewhere, 
while a few grow fairly well, but not at their best, under different condi- 
tions beyond these limits. Taken as a whole, the plants of the flood 
plain find there their real home, and they exhibit—such of them, at least, 
as have been carefully observed with reference to this—a striking con- 
formity of the root-system to the peculiarities of the soil in which they 
are growing. This, as already stated, is of fine texture, retentive of 
moisture, and of great depth, with the water-table varying in level, but 
apparently never beyond the reach of the long tap-roots of the mesquite 
and Acacia. 

The root-system of these plants consists of a tap-root which grows 
rapidly downward, and when developed is always within reach of a per- 
manent, deep water-supply, and a system of widely spreading lateral 
roots which are in relation to more superficial layers of the soil. Thus 
the plant is admirably fitted to absorb water largely from the upper 
layers when these are moist, and at the same time, and also in times of 
drought, without any interval of precarious supply, to draw on the deeper 
sources. below. The contrast between this and the shallow root-systems 
of many of the great trees of eastern mesophytic forests, familiar to 
everyone who has seen them uprooted by heavy winds, is highly instruc- 
tive. There is little wonder that the mesquite and Acacia constricta 
have tenaciously held their places through all vicissitudes and promise 
to be dominant in their habitat until actually rooted out. 

There are evident movements of vegetation now taking place within 
this association. Bzigelowia hartwegu, a native weed, has spread with 
wonderful rapidity under the mesquite within the last three years of 
favorable seasons; and other species, both weeds and useful plants, have 
been brought in at various times and by different agencies to such an 
extent as to give to the vegetation, in places, a distinct change of char- 
acter. It should be added that Bigelowia (and presumably other low- 
growing plants of the flood-plain) has a far less extended root-system 
than the mesquite, obtaining water from relatively near the surface. 
If the root-system does not reach to the water-table, and this can hardly 
occur, it would seem that this plant, and others of this association of the 
same physiological class, must be more xerophytic in habit than the mes- 
quite. It seems that the flood-plain may be thought of as the home of 
two quite different sets of plants, one with a tap-root and its branches 
reaching the water-table, and the other depending upon the water con- 
tained in upper layers of the soil; the former approaching mesophytic, 
the latter more definitely xerophytic habits. 


PLANT ASSOCIATIONS AND HABITATS. 13 


(4) SALT=SPOTS; ASSOCIATION OF SALT=BUSHES. 


The description already given holds in all its general features for the 
soil and vegetation of the flood-plain of the Santa Cruz River, but cer- 
tain areas, some of which approach the neighborhood of Tumamoc Hill, 
are so exceptional in character as to call for a special account. 

The so-called salt-spots include areas varying from a few rods to many 
square miles in extent (plate 5), and are widely distributed throughout 
the arid and semi-arid regions west of the Rocky Mountains. As they 
occur in the Santa Cruz Valley, and also in the valleys of the Gila and 
Salt Rivers in Arizona, they are areas of low ground, insufficiently drained, 
in which have accumulated alkali salts, especially salts of sodium, and 
which are conspicuously marked both by the presence of various species 
of Chenopodiaceze (‘‘salt-bushes’’), and also by the absence of many 
species characteristic of surrounding areas. 

In the lowest part of such spots, where the drainage is most defective, 
Sueda moquini is characteristic, and in some cases is well-nigh exclusive 
in its occupation of the soil. Commonly, however, there are associated 
with it a few other plants, especially one or more species of Atriplex. 
Thus, Alriplex lentiformis grows luxuriantly, even where the so-called 
black alkali, chiefly sodium carbonate, forms a heavy crust, and both 
Atriplex nuttalia and A. elegans are of frequent occurrence in low spots 
where the accumulation of alkali has reached a high percentage. 

Beyond this association, and outside of it, Atriplex canescens, in many 
cases, is conspicuously present. Its range, however, extends over the 
flood-plain and even beyond it, so that although closely related geneti- 
cally, it can not be referred to the more restricted association of salt- 
bushes which have the salt-spots as their habitat. Thus a zonal arrange- 
ment, often well marked, is produced, in which the center may be quite 
bare of plants, while around it, in successive concentric zones, are (1) 
Sueda moquint and Atriplex nuttallit, (2) Atriplex polycarpa, (3) Atriplex 
canescens and various other species which belong around or outside the 
limits of the salt-spot proper. 

It has been shown by Cannon (1908) that at the salt-spot on the flood- 
plain of the Santa Cruz, just north of Tucson, where there are three well- 
marked zones of vegetation, alkali salts are most abundant in the inner, 
_least abundant in the outer, and intermediate in amount in the inter- 
mediate zones. 

In view of their persistency in such habitats, and in a definite order 
corresponding to amounts of alkali, it seems well-nigh self-evident that 
Sueda and several species of Atriplex must be regarded as true halophytes 
or “‘salt-loving’”’ forms; that is, as having become specially adapted to 
soils containing a large percentage of alkali salts. It is true that in the 
vicinity of irrigating ditches the species characteristic of salt-spots, and 


14 DISTRIBUTION AND MOVEMEN’S OF DESERT PLANTS. 


especially Sueda moquint, grow more luxuriantly than in the salt-spots 
themselves, but this is a physiological response characteristic of plant we 
in general, and can hardly be taken in evidence. 

The question of soil preferences involves great difficulties, due in part 
to the apparently contradictory deportment of many plants that have 
been cited in evidence, and still more to lack of sufficiently extended 
observation and experiment. The interesting experiments of Kearney 
and Harter (1907) on the comparative tolerance of various plants for the 
salts common in alkali soils shows clearly the great complexity of the 
subject when the relation of any plant to a mixture of alkali salts is under 
investigation. On the other hand, an illuminating discussion of the 
soil preferences of alpine plants by Fernald (1907) has brought forward 
so many incontrovertible facts, established by years of careful observa- 
tion, that it no longer seems possible to doubt that the chemical com- 
position of underlying rocks has been an important factor in determining 
the distribution of the remarkable ecological group to which his study 
has been directed; the whole question, however, calls for experimental 
investigation. 


(5) THE WASH; PALO VERDE=-CATCLAW ASSOCIATION. 


At the foot of Tumamoc Hill, on the northwest, there is a ‘‘ wash,” 
formed by the union of a number of similar ones which come down from 
the long slopes to the westward, the system presenting throughout its 
extent the usual features of a dry watercourse, with sandy bottom, along 
which a dense vegetation of shrubs or low trees marks its course even 
when seen at a distance (plate 5). The wash with its branches consti- 
tutes a drainage system for the west slope of Tumamoc Hill on the one 
hand and the east slope of the nearby hills of the Tucson Range on the 
other, forming the natural channel or run-off into which water runs or 
seeps from the adjacent banks. Though its bed is usually dry, there 
are abundant indications of the presence of water below the surface, 
especially in the character and habits of its vegetation. In the first place, 
certain species, notably the creosote-bush, attain here a size and vigor 
of growth in striking contrast with the low, straggling individuals of the 
slopes nearby; and, secondly, other species, particularly the mesquite, 
everywhere taken as an indicator of water, are conspicuously present. 
Both of the species just named are prominent constituents of the vege- 
tation of these washes, but since their characteristic habitats are else- 
where, other species have been chosen to give their name to this asso- 
ciation, viz, the palo verde (Cercidium torreyanum) and catclaw (Acacia 
greggit). 

At present, as the process of base-leveling slowly proceeds, these 
species are advancing from the flood-plain toward the adjacent hills. 
Cercidium torreyanum (plate 6) is already found far up one of the gulches 


SPALDING atte 


~~ 





Salt-spot in Salton Basin. Clumps of Allenrolfia near the center and masses of Alriplex 
nearer the margin, as seen at the left. This area, now covered by water, was photo- 
graphed before the last invasion of the Salton Sea. 





The wash northwest of Desert Laboratory. Lining its banks are Cercidium torreyanum, 
Acacia greggii, A. constricta, Prosopis velutina, and Celtis pallida. 


CAMPBELL ART CO., ELIZABETH, WN. J, 


THE LinPARy 
OF 1 HE 


UichRs) GF ILLinors 





SPALDING 


PEATE 6 





> 
eine ae 


Single individual of Cercidium torreyanum. Branching opuntias in background. 






a 
i 
a 
pete SO BR Or 0, 
* ee We , 
Tame eS ytd 
AO eekis 


Ephedra trifurca in wash west of Desert Laboratory. 


CAMPBELL ART CO., ELIZABETH, N. J. 





PLANT ASSOCIATIONS AND HABITATS. 15 


of Tumamoc Hill. The catclaw deports itself in the same way. Ephedra 
trifurca (plate 6), another characteristic species, ranges still more widely, 
approaching the habits of the creosote-bush in attaining its best develop- 
ment where there is an abundant water-supply, but, like the latter, capable 
of maintaining itself successfully where the water content of the soil is 
lower, especially on light, more or less sandy soils. 

A few other woody species find in this habitat a congenial home. Con- 
dalia spathulata attains here its best development. Condalia lyciordes, 
a companion of the mesquite, advances with it up the washes, and Celtis 
pallida, here a plant of the cliffs, also occurs in the wash, absolutely 
avoiding the intervening slopes, thus presenting further evidence, if such 
were needed, that soil-water is a factor of prime importance in deter- 
mining the distribution of this association of plants. 

Five of the species that have been named as members of this association _ 
have been observed by Blumer (1908) to change their topographic loca- 
tion with altitude. These are Cercidiwm torreyanum, Acacia greggit, 
Prosopis velutina, Condalia lycioides, and Celtis pallida. At Tucson, at 
an altitude of 2,200 to 3,000 feet, all of these inhabit the washes, and 
with a single exception (Celtzs pallida) are hardly met with at all else- 
where; but at higher elevations, 3,500 to 4,500 feet, in the neighboring 
mountains of the Tucson Range, the Santa Catalinas, and the Rincons, 
they are found spreading out on gravelly and other upland soils, no longer 
confined to washes, and deporting themselves as ordinary members of 
the shrubby upland growth there prevalent. Taking these higher eleva- 
tions as the point of departure, it is found that even 1,000 feet lower all 
these species exhibit a marked tendency to confine themselves to water- 
courses or, at all events, to places where there are good conditions as 
regards soil-moisture and some degree of protection from the more extreme 
desert conditions prevailing at the lower levels. The single individual 
of Yucca elata, a plant of higher levels, that has been found on the Lab- 
oratory domain, is at the edge of the wash, protected by the higher 
vegetation around it. 

It should again be noted, as already suggested, that many of the char- 
acteristic species of the habitats thus far discussed, though exhibiting 
more or less plainly certain structural features distinctive of xerophytes, 
are semi-mesophytic or mesophytic, in some cases even hydrophytic, as 
regards soil relations. The willows and cottonwoods of the river-banks 
are, in their habits, what they are the world over. The mesquite of the 
flood-plain sends its roots down to the water-table, and elsewhere is 
restricted in its range to habitats in which a satisfactory water-supply 
is at hand, and the palo verde and catclaw of the washes evidently have 
much the same dependence on a sufficient amount of soil-water. 

The same thing appears to be true of various species of salt-bushes, 
which follow watercourses, or—in the salt-spots—occupy places of seep- 


16 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


age. In view of their habits, it may fairly be questioned whether such 
xerophytic structures as some of the salt-bushes exhibit stand primarily 
in relation to “physiologically xerophytic”’ soil conditions or not; in 
fact, it seems not at all improbable that they may be found to fall into 
the same category with the mesquite and catclaw, exhibiting certain 
xerophytic structures which are useful in the dry air of the desert, and 
which also in cases of extraordinary drought successfully regulate tran- 
spiration and make less draft on the resources of the plant. 


(6) THE MESA-LIKE SLOPES. 


(a) CREOSOTE-BUSH ASSOCIATION. 


The long, gentle slop2 of the “mesa,” as it is commonly but incorrectly 
called, rises from the flood-plain, at first with an almost imperceptible 
grade, becoming steeper as the ascent continues to the hill above. Its 
soil, as already stated, is mainly gravelly or sandy, with but little loam 
and with a considerable amount of caliche. ‘The slope and the nature 
of the soil insure perfect drainage and aeration. But with the poor soil, 
with relatively little capacity for the retention of water, it is plain that 
we have passed, almost at a step, from soil conditions that may be classed 
as mesophytic, or semi-mesophytic, to those that are suited only to dis- 
tinctively xerophytic plants, such as grow where peculiarly trying con- 
ditions prevail. Under these conditions, as might have been anticipated, 
nearly all of the perennial species of the flood-plain abruptly cease; in 
fact, the creosote-bush is the single one that. is capable of successfully 
holding the ground on the worst places, and even this is dwarfed in the 
struggle to maintain existence where the water-supply is never abundant 
for any length of time, and through a large part of the year is necessarily 
meager in the extreme. Yet, in spite of these untoward conditions, the 
Larrea claims this zone as its own, and has produced upon it an almost 
pure growth of a single species (plate 8). 

It is impossible, as yet, to enumerate completely the peculiarities of 
this plant, structural and physiological, which have enabled it, more 
successfully than any other, to cope with the worst of desert conditions 
as they exist in the Southwest. Some of these, however, are obvious. 
It is provided with a root-system (plate 7) which both penetrates deeply 
and also branches widely near the surface, thus being in a position to 
avail itself of water that accumulates at either level; it is capable, as few 
other species are, of absorbing appreciable quantities of water through 
its leaves, and these are admirably protected by varnish and in other 
ways against excessive transpiration. With the coming on of drought 
a large percentage of its leaves are thrown off and a still further reduction 
of surface is accomplished by the leaves becoming checked in their devel- 
opment long before they have attained their full size, as seen in individ- 


SPALDING 


PLATE, 7 





: Creosote-bush a year old, showing comparative extent of top and root-system. 





Vegetation of the wash. In foreground Yucca elata. 


“© 


CAMPBELL ART CO,, ELIZABETH, N. J. 





SPALDING 


PLATE 8 


ty 





Zone of creosote-bush at the foot of Tumamoc Hill on the north. In the background Santa 
Catalina Mountains beyond the flood-plain of the Santa Cruz River. 


tf 





Tucson slope beyond the wash west of Desert Laboratory. Franseria deltoidea and Opuntia 


fy fulgida constitute the characteristic vegetation at this point. 


CAMPBELL ART CO., ELIZABETH, N. J. 


i 


THE Lippapy 
OF ine ea 
ARIVERSHTY gp Ebuors ae 





PLANT ASSOCIATIONS AND HABITATS. YW 


uals of the slopes compared with those of the flood-plain and washes. 
Its mode of branching and of reproduction from the root is also advan- 
tageous. Whole systems of branches die, and thus permit their share of 
water from the soil to go to others, and if, under extraordinary stress, 
the whole top perishes, fresh shoots from the root come up upon the 
return of rain, and vigorous growth again takes place. Fitted in so 
many ways, and probably in others less obvious, to cope with the vicis- 
situdes of its habitat, the Larrea holds complete sway in this zone, and 
from Texas to California the broad belt of creosote-bush, covering’ the 
long slopes that form the approach to the mountains at different altitudes, 
presents a most striking and characteristic feature of the landscape. 

A few other species are of occasional occurrence in this special habitat 
of Larrea. ‘Thus on the slope at the foot of Tumamoc Hill a very few 
individuals of Fouquierra splendens are established, and in places Opuntia 
julgida is frequent; none of these, however, approach the creosote-bush 
as the universal and distinctively characteristic species of this zone. 

As the slope advances upward with the long continuance of disinte- 
gration and erosion, the creosote-bush advances with it, and the presence 
of this plant, both as the successful pioneer and the final possessor of 
the soil, may be seen almost everywhere throughout its range. Thus 
we have the interesting case of a well-marked habitat, with a single spe- 
cies forming in it, at least in places, a perpetually renewed close asso- 
ciation of its own. 

The description just given applies to the slope as seen near the foot 

of Tumamoc Hill and in situations such as the “alluvial fans,’’ where 
- the conditions of slope and soil characters are essentially similar. There 
are wide areas commonly included under the term “mesa”? upon which a 
greater variety of conditions, with corresponding differences of vegetation, 
prevails. An example of this is the ground included in the Laboratory 
domain lying to the west of the wash, presently to be described. 


(b) FRANSERIA ASSOCIATION. 


Beyond the wash, on the Tucson slope to the west of the laboratory, 
and elsewhere on the domain, areas of considerable extent occur where 
there is a soil of coarse texture, approaching gravel, on which /’ranseria 
deltoidea and Opuntia fulgida constitute the greater part of the vegeta- 
tion. ‘he former is often very numerously represented (plate 8), and in 
many places is not accompanied by the cholla, but where the two attain 
their best development they are likely to occur together. I*requently 
Krameria canescens and a few other species are associated with them. 
As far as appears from present evidence, this association is one that is 
determined first of all by soil relations. ‘These are described in a later 
paragraph. 


18 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. _ 


(7) THE HILL. 


Above the Larrea zone the hill is the habitat of a large number of con- 
spicuous and characteristic perennials, which, although they may be 
classed together as a single association, differ among themselves to such 
an extent in ecological traits and choice of habitat as to call for consid- 
eration under separate groups. 


(a) PLANTS OF GENERAL OCCURRENCE ON ALI, EXPOSURES; ASSOCIATION 
OF FOUQUIERIA AND PARKINSONIA MICROPHYLLA. 


The species of this group, more than any others, give its character to 
the vegetation of the hill as a whole. Growing as they do equally well 
on all exposures, to differences of which members of other groups show 
remarkable sensitiveness, they form at once the most widely spread and 
most typical representatives of the vegetation of Tumamoc Hill (plate 
9). Their adjustment to a wide range of physical conditions seems 
well-nigh perfect, yet it is exhibited in widely different ways. The habits 
of Fouguierva and Parkinsonia, to go no further, are very different, as 
regards the production and fall of foliage leaves, arrangements for pro- 
tection, and other ecological characters, but they grow side by side on all 
exposures and on various soils, so that, however limited some of them 
are as to geographical range, it is plain that within its limits they have 
attained a high degree of adaptation to widely varying soil and atmos- 
pheric conditions, and are to be reckoned as highly successful desert 
species. 


(b) PLANTS OCCURRING ON SOUTHERN EXPOSURES AND LARGELY WANT- 
ING ON NORTHERN ONES; ASSOCIATION OF CEREUS GIGANTEUS AND 
ENCELIA FARINOSA. 


The species just named are closely and widely associated and are both 
far more numerous on southern exposures, as well as eastern and western, 
than on northern ones. ‘This is strikingly shown on the two sides of 
the gulch adjacent to the Desert Laboratory on the southwest. On the 
right side of this gulch, with a southern and partly western aspect, there 
are upwards of 70 sahuaros and the ground is almost covered with a flour- 
ishing growth of Encelia, numbering thousands of individuals (plate 
9). The left bank, on the other hand, with its generally northern aspect, 
has less than a dozen sahuaros and only a few scattering groups of Encelia 
(plate 10). ; 

Observations of temperature and humidity and experiments with 
seedlings have been in progress for some time, with a view to determining 
as far as possible the factors to which this striking limitation as regards 
local habitat is due. (See p. 44 et seq.) 


SPALDING PLATE 9 





* 
‘ 
Y 
Slope of Tumamoc Hill, showing a strong growth of characteristic vegetation in which 
Fouquieria splendens and Parkinsonia microphylla are conspicuous. 
e 
s 





Right side of gulch near Laboratory, with generally south exposure. 


CAMPBELL ART CO., ELIZABETH, NW. J. 





PLANT ASSOCIATIONS AND HABITATS. 19 


(c) PLANTS OCCURRING ON NORTHERN EXPOSURES, BUT WANTING ON 
SOUTHERN ONES; LIPPIA ASSOCIATION. 


On the left side of the gulch just referred to, and near its head, Lippia 
wrighttt forms an extensive patch (plate 10), and in the vicinity of Tuc- 
son it frequently occurs on similar northern exposures, never, as far as 
observed by the writer, on a southern exposure; but in the Chiricahua 
Mountains and elsewhere, as shown by Blumer (1908), it exhibits a 
marked variability in choice of aspect as governed by altitude; thus, at 
elevations of 3,000 feet and under it grows only in protected places of 
north aspect, but loses its aspect preference at altitudes approaching 
5,000 feet, while at those near 6,000 feet it is definitely limited to southern 
exposures. It appears, then, that this plant completely changes its aspect 
preference within a range of not more than 3,000 feet. At the lower 
levels it requires protection from too severe desert conditions, and finds 
this on sheltered rocks of northern aspect. At the higher altitudes, appar- 
ently requiring protection from cold, it finds a congenial home on warm 
southern exposures, while at intermediate elevations, in the neighborhood 
of 5,000 feet, the extreme conditions of both higher and lower altitudes 
are so far modified that it grows on all exposures. 

Various other species show at this place a distinct preference for the 
northern exposures of the hill. Among. these are Abutilon imcanum, 
Brodiea capitata, and many others, annuals even more conspicuously 
than perennials. It is evident, however, from the facts cited in the case 
of Lippra, that this choice of aspect is not necessarily constant, and that 
direction of slope determines distribution merely as it presents a combi- 
nation of conditions which may change so greatly, even within a few 
miles, as to completely reverse the aspect preference of a given species. 


(d) PLANTS OF THE CLIFFS; HYPTIS-NICOTIANA ASSOCIATION. 


One species of each of the genera just named is found growing almost 
exclusively on the abrupt cliffs of Tumamoc Hill and on similar cliffs 
elsewhere. Celtis pallida, though by no means confined to the cliffs, is 
of common occurrence on them, where it evidently finds a congenial 
home. ‘The factors concerned in determining the choice of habitat exhib- 
ited by members of this association are not known. It may well be that 
where there is so little soil the result may in part be the outcome of com- 
petition, but regarding this point we are without positive knowledge. 
Hyptis and Nicotrvana are rarely seen growing elsewhere, though the 
latter is sometimes found growing in the sandy soil of washes, and in this 
latter habitat Celtis is of rather frequent occurrence. I am disposed to 
think of water-supply as again the main factor. There are probably 
pockets or fissures in the rocks where Hyptis and other plants obtain 
what they need, and the occurrence of Celtzs and Nicotiana in washes 
looks to this also. 


20 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


(8) SUPERFICIAL SOIL LAYERS; ASSOCIATION OF ANNUALS. » 


Thus far, attention has been directed wholly to perennial species which, 
rooted in their places and remaining there for a period of years, constitute 
the more permanent and conspicuous elements of the vegetation. There 
are, however, a very considerable number of annuals which, at certain 
seasons of the year, become a highly characteristic and conspicuous 
constituent, and whose habits and distribution involve interesting and 
important problems. As in the case of the general association of per- 
ennial plants of the hill, we are here dealing with a more or less composite 
biological group, which, however, covers a wider area, and is perhaps 
even more influenced by distinctively local conditions. This, in its turn, 
will be studied to best advantage under several groups, which, though not 
sharply delimited, are sufficiently well defined. Taking the winter annuals 
first, two divisions based on physiological requirements are recognized. 


(@) SHADE-LOVING SPECIES. 


A good representative of this group is Parietaria debilis, which grows 
luxuriantly in the continuous shade of rocks on northern exposures. The 
water-content of this plant is very high, the stems having the shining 
pellucid appearance of the eastern clearweed, and there are no indications 
of specially developed means for the prevention of excessive transpira- 
tion. Though designated as shade-loving, because of its almost exclusive 
occurrence in the deep shade of rocks, there is, nevertheless, evidence 
‘that its growth here is conditioned by moisture, rather than lessened 
light intensity. It ventures here and there a little beyond the shade, 
and though strikingly modified in form and structure by exposure to 
full insolation for a part of the day, it is nevertheless capable in this 
situation of maturing its seeds. It is apparently a plant of essentially 
the same physiological requirements as the so-called shade-loving species 
of mesophytic forests, and in the one case as in the other jt may well be 
questioned whether, in the complex of physical factors necessarily in- 
volved, light intensity plays as important a réle as has been attributed 
to it. In any case, it is to be understood that the classification here 
adopted is retained chiefly for convenience in discussing the observed facts. 

A number of species of winter annuals grow luxuriantly on northern 
slopes and in the shade of rocks and bushes that are hardly met with on 
full southern exposures. Such are Bowlesia lobata and, to a less marked 
degree, species of Phacelia, Amsinckia (plate 11), and others. study of 
the habits of these plants, however, indicates, quite as plainly as in the 
preceding case, that the water relation is of primary importance. Bow- 
lesta, for example, which has grown thickly in the shade of a creosote- 
bush or palo verde, continues, when they are destroyed, to produce new 
crops year after year on the same ground, where the accumulation of 
humus insures a better water-supply. This and the other plants just 


SPALDING PLATE 10 





Left side of gulch near Laboratory, with generally north exposure. 





Lippia wrightii forming a nearly pure growth on the left side of the gulch, near its head. 


CAMPBELL ART CO., ELIZABETH, N. J. 


THe gipeary 
OF i#é 


CNSISAaITY gf ILLnars 





PLANT ASSOCIATIONS AND HABITATS. a 


named are entirely capable of successful growth in the full light of the 
sun, provided the root-system is still in soil that supplies a sufficient 
amount of water. The light relation, notwithstanding the apparent choice 
of shade on the part of these species, appears, therefore, to be subordinate 
to water-supply as a factor determining local distribution. 


(b) LIGHT-LOVING SPECIES. 

Many of the winter annuals of Tumamoc Hill grow everywhere in the 
open, where they are fully exposed to the sun, and to a greater or less 
extent, also, where there is some shade; such are species of Harpagonella, 
Pectocarya, Plantago, Daucus, Erodvum, and other genera. ‘These also are 
far more numerously represented on northern than on southern expo- 
sures, but from the fact that the range of light intensity under which 
they habitually accomplish their normal development is so great, there 
can hardly be any doubt that the local distribution of these species, like 
that of the preceding sections, is correlated first of all with water-supply; 
and far less, if indeed to any appreciable degree, with light intensity. 

In its ultimate analysis, therefore, the local distribution of the winter 
annuals here represented is fundamentally based on the water relation. 
So broad a conclusion should receive confirmation from the determination 
of physiological constants involving a prolonged series of experiments, 
but the appearance or non-appearance of both these and the summer 
annuals is so obviously related to rainfall that any other conclusion as 
to their relation to water-supply is, with present evidence, impossible. 

The summer annuals appear after the summer rains, during the period 
of the highest temperature of the year. Once the ground has become 
wet they germinate and develop with great rapidity, some of them grow- 
ing to a large size and in such numbers as to form, in many places, a 
conspicuous feature of the vegetation. They bear the same relation 
to summer rains that the winter annuals do to winter rains, and their 
behavior necessitates the same conclusion as to the fundamental impor- 
tance, in their case as in that of the winter annuals, of the water relation. 

Not less certain, however, is the fact that the habits of both are directly 
correlated with differences of temperature. It is impossible, for example, 
to induce the germination of the seeds of winter annuals in summer tem- 
peratures, unless they have previously been subjected to a low temper- 
ature. Thus while water-supply, with annuals as with perennials, is a 
chief factor in determining distribution in space, differences of temperature 
determine with remarkable precision and certainty the distribution in 
time of the winter and summer annuals, and the face of the landscape 
is twice a year changed to correspond with their biological habits. It 
results that areas which are densely covered with winter annuals in 
February and March afford ample room and all the necessary conditions 
of development to the luxuriantly growing summer annuals of August 
and September. 


22 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


(9) PARASITIC AND SYMBIOTIC PLANTS. 


Though not forming associations properly comparable with the fore- 
going ones, the parasitic plants of the Laboratory domain call for notice. 
Two species of mistletoe are widely distributed in this region, one of 
which (Phoradendron californicum) is of fairly frequent occurrence on 
Tumamoc Hill and the adjacent valleys, where it grows on Prosopis 
velutina, Parkinsonia microphylla, Cercidium torreyanum, Acacia con- 
stricta and A. greggu (plate 11). Of these P. microphylla and the mesquite 
are its most frequent hosts on the domain, where it has been observed 
but once on A. constricta, and once on Cercidium torreyanum. The other 
species, Phoradendron flavescens, has not been found within our limits, 
but it is of common occurrence in the valley of the Santa Cruz River 
in the vicinity of Tucson, growing on cottonwood and ash trees. 

In contrast with the limited number of hosts which are here affected 
by these species of mistletoe, Mr. J. C. Blumer states that in the Chiri- 
cahua Mountains, at altitudes of 5,000 to 6,000 feet, he has observed 
Phoradendron flavescens growing on two species of ash, on sycamore, 
on two species of cottonwood, and on two of willow, and in its form 
pubescens on many oaks. This is as might be expected from the far 
greater variety of hosts and the greater range of environmental con- 
ditions in the mountain region. 

Important observations and experiments have been made on both 
species by Cannon (1904), to which bare reference can here be made. 
These relate to germination and mode of penetration of the host and to 
the rate of transpiration of the parasite as compared with that of the 
host. In regard to the latter it was found that in the various experi- 
ments undertaken the parasite transpired more rapidly than the host, 
from which there results an unnatural draft on the resources of the host, 
which is often followed by the death of the infested branches, the con- 
sequent great disfigurement of the tree, and, in some cases, its ultimate 
destruction. It does not seem, however, that any of the hosts affected 
have been appreciably limited in their distribution by the action of these 
parasites. As for the distribution of the parasites themselves, it appears 
from the observation of the author referred to that it takes place from 
tree to tree by the agency of birds, but its distribution in a tree is such 
as to indicate that when once the higher branches have been infected, 
in due time seeds from this source reach the branches below without 
the agency of birds. 

An exceedingly interesting case of parasitism, in which a large number 
of hosts is involved, is that of Orthocarpus purpurascens palmeri, which 
in springtime grows so thickly on the sides of Tumamoc Hill as to give 
them a reddish tint noticeable at considerable distances. Dr. Cannon 


has identified some 20 species of plants on the roots of which this parasite 
fastens. 


SPALDING PLATE 11 


Fe 
- ‘ed 





Rank growth of winter annuals, at this place chiefly Amsinckia spectabilis. 


CAMPBELL ART CO., ELIZABETH, N. J 












Me 4; 





+ 
> ° 
mn 4 
: 
a s 
j Ae coe c 
i] = a i 7 
ae 7 * .% 
ie. es 
7 arte tits vce 
> —— 
‘ ® 









ee | | THE LIBRARY 
| >) SEA, | 
uiyenstty GF UNOS thats 





PLANT ASSOCIATIONS AND HABITATS. 26 


As far as present observations extend, the fungus flora of the Labora- 
tory domain may be characterized as insignificant. A few parasitic forms 
of the Perisporiacee and other groups occur from year to year on certain 
annuals, and a few saprophytes have been found on bark, and, in two or 
three instances, growing in the low ground of the wash. Undoubtedly 
careful search would result in the collection of a considerable number 
of species, but the fact remains that the fungus flora here is extremely 
limited, as it would seem it must be from the severe conditions imposed. 
This meager showing is again in strong contrast with the rather rich 
display of fungi reported by Blumer (1908) in the Chiricahuas, and prob- 
ably matched in other neighboring mountain regions where favorable 
conditions prevail. | 

Better developed, and of more ecological interest, are the symbiotic 
plants constituting the lichen flora, an account of which is given on pages 
2AOs2 7. 


24 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. _ 


LICHENS OF THE DESERT LABORATORY DOMAIN.! 


The lichens of the Laboratory domain form a remarkable assemblage 
of plants. Irom collections made by J. C. Blumer and V. M. Spalding, 
32 species have thus far been identified, but many more species are certain 
to be found when the ground is more exhaustively worked. ‘The collectors 
were asked to find any loosely foliose or fruticose lichens, and only a single 
loosely foliose species was sent and not a single fruticose one; the former, 
moreover, was possessed of very special adaptations. But very few were 
collected from the soil, although they were frequently looked for, and 
repeated requests for material from woody plants brought nothing but 
a few sterile and poorly developed specimens of a Physcia and a Placodium. 
These were collected on Parkinsonia microphylla very close to the ground, 
among rocks on a north slope. 

Of the 32 species 24 were found on rocks and, as shown by Fink (1899, 
1904), bear a striking general resemblance to those occurring in regions 
of greater rainfall on exposed granite and southward facing riprap, which 
represent xerophytic habitats. 

One may look through the whole list of 24 lichens growing on the rocks 
of Tumamoc Hill without finding more than 4 species with conspicuously 
lobed thalli. These are Placodium elegans, P. murorum, Lecanora murals, 
and Parmelia conspersa; and these when compared with lichens of the 
same species from more moist climates show, as a whole, a perceptible 
shortening of the lobes of the thalli. This condition of affairs may stand 
in relation to the dry air of the desert, or possibly to the high winds which 
prevail there. It is well known that the more a thallus is branched or 
lobed, the more young, tender, growing points are exposed, and the 
greater the amount of transpiration of moisture. It is also true that 
many fruticose lichens, certain Evernias, for example, notably lacking 
on Tumamoc Hill, can scarcely maintain themselves in open places, 
where they are subjected to strong gales, but seek protected habitats, 
where they will not be torn from their substrata. These forms, moreover, 
are usually conspicuously branched and present much surface and many 
tender, growing areas to the drying effects of wind and a dry atmosphere. 
The general form, therefore, of the rock-inhabiting lichens of the Labora- 
tory domain is advantageous from either of these points of view. 

In general, the 24 lichens collected on the rocks of Tumamoc Hill are 
protected by some sort of mechanical device, usually a definite pseudo- 
parenchymatous cortex, and inclosed, dead algal cells, which protect the 
living algal cells and the fungal hyphe of the medullary layer against 
the drying effects of high winds and the direct rays of sunlight. Zukal 
(1895) has observed that the cortex is thicker in certain lichens growing 
in places where they are exposed more than usual to intense light and 
dry atmosphere than in the same species in less-exposed positions. One 
of the most helpful studies in connection with the present problem would 
be the comparison of some of the lichens of Tumamoc Hill with repre- 
sentatives of the same species from regions having average conditions 
of light, moisture, temperature, and wind, with reference to develop- 
ment of cortex. 

_ There should also be a more thorough study of the presence and func- 
tions of coloring-matter in the cortex than has vet been made. Of the 





* Abstract from paper prepared by request and contributed by Bruce Fink, Ph. D., 
Professor of Botany in Miami University. 


PLANT ASSOCIATIONS AND HABITATS. 25 


rock-inhabiting lichens of Tumamoc Hill every species having a light- 
colored thallus shows a more or less evident development of black lines 
or spots on the upper surface. These lines or spots are so numerous 
on older portions of some of the thalli as to darken, more or less, the 
otherwise light-colored surface. Zukal, in his discussion of the protec- 
tive significance of colors in lichens, speaks of such lines of black as occur- 
ring on younger or injured portions of thalli to protect the algal cells 
from the intense rays of the sun in hot regions; but the writer has found 
the lines and spots better developed over older parts and could not detect 
their unusual development in connection with cracks of the thalli. In 
the unsettled state of our knowledge regarding protective coloration, it is 
very desirable that careful observation should be made of the relation 
of development of the black areas to the position of the particular plant 
on the rocks with reference to the sun’s rays. The study of the relation 
of development of coloration at the tips of the branchlets of Parmelia 
conspersa to the orientation of these branchlets to the sun’s rays would 
be specially instructive. 

The 15 genera thus far collected have been subjected to careful exam- 
ination with reference to the structure of the thallus and its relation to 
xerophytic conditions. The Acarosporas, which form a large proportion 
of the rock-inhabiting species, are cellular throughout, so that the algal 
cells are unusually well protected against too intense light or too much 
transpiration of moisture, or both. The same is true of the Endocarpis- 
cums. The Lecanoras have either an upper cellular cortex or a pseudo- 
cortex of entangled hyphz, and the Placodiums show similar structure. 
The one Parmelia is a very closely adnate species, which is found to 
possess a stronger cortex than the closely related Parmelia caperata, which 
usually grows in less xerophytic conditions. The Dermatocarpons are 
all well protected by strong cortices, and Dermatocarpon miniatum, the 
only species not closely adnate, is attached to the rocks by a very strong 
umbilicus, while the lower cortex is so strongly developed that no ordi- 
nary wind can tear the plants from the rocks. Not to enumerate the 
remaining genera, most of which can not be ranked as important floral 
elements, the general statement. may be made that structurally the lichens 
of the Laboratory domain are well protected both against the danger 
of excessive transpiration and that of being torn away by high winds. 

The lichens sent for study were collected from 7 stations. They are 
most numerous on the north side of the dark-colored basaltic rocks which 
compose so large a part of Tumamoc Hill, and are very poorly represented 
on southern exposures and on the tuff which has been quarried in a few 
places for building purposes. At station III, which is a very steep slope 
on the south side of Tumamoc Hill, facing directly south, at an altitude 
of 2,700 feet, the lichen habitat consists of loose blocks of tuff and basalt, 
where nothing in the way of seed plants exists but a few creosote-bushes. 
Only 7 species of lichens were collected here, 5 of which belong to the 
genus Acarospora. The frequent occurrence of members of this genus on 
southward-facing riprap and their very rare occurrence on the northward- 
facing riprap a few feet away has been noted elsewhere by Fink (1904, 
p. 278) and, with the data obtained at station III, establish beyond doubt 
that the Acarosporas, with their strong, protective cortices and their 
cellular structure throughout, are the most characteristic xerophytes of 
all our American lichens hitherto studied from the ecological point of 
view. Tuff is a drier rock than basalt, not holding water so well, and, 


26 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. - 


corresponding to this drier substratum, not only at station III but at 
other points, the Acarosporas are well represented upon it. 

Of atmospheric conditions likely to influence the distribution of lichens, 
relative humidity and air movements are undoubtedly the most important 
factors. The relative humidity is known to be as low as 8 per cent of 
saturation at the Desert Laboratory at times of special dryness in summer, 
and it varies from this to a high percentage during the rainy seasons. 
The drying winds from the east, south, and west, day after day, doubtless 
interfere to some extent with the development of lichens on these three 
sides of rock exposures, accentuating the effect of direct sunlight, thus 
leaving the northward-facing ledges by far the best habitats. 

It has been shown by Spalding (1906) that certain desert seed-plants 
absorb more or less water through their leaves and young shoots, some of 
them as much as 1g per cent of their weight, and he has also found that 
some species absorb a very small amount of water-vapor from a nearly 
saturated atmosphere, through their leaves and twigs. It is thought that 
this absorption of water and water-vapor through subaerial parts may 
be of some slight advantage. Jumelle (1892) has experimented with 
lichens in somewhat similar fashion. His experiments show that lichens, 
compared with seed-plants, contain very little water at any time, but 
that the former are much more able to absorb water or water-vapor 
in the proportions needed than are the latter, according to the results 
obtained by Spalding. 

Aside from the gelatinous species, lichens need but little water and 
are able to obtain all that is required through the general surface, instead | 
of through specialized organs, as roots. It may be assumed from these 
investigations, until otherwise proven, that lichens are able to absorb 
at least a large proportion of the moisture needed directly from water- 
vapor of the atmosphere and from water falling upon them in the form 
of rain. As far as appears, however, the statements of Jumelle do not 
rest upon experiments made on any desert lichens, and similar investiga- 
tions of some of the lichens about the Desert Laboratory would be cer- 
tain to yield instructive results. 

Experiments such as those made by the authors referred to, performed 
upon the crustose lichens of the desert in the driest condition in nature 
and giving the relation between fresh and dry weight, would give data 
regarding the amount of moisture retained in lichen thalli during the 
driest times. Accompanying this should go observations regarding the 
length of time that these lichens may be kept dry and then resume active 
respiration and assimilation on the return of favorable conditions. Since 
lichens have no special storage organs, nor organs of absorption extending 
any considerable distance into the substratum, the retreat of the evap- 
orating surface into the soil leaves any lichens growing upon exposed soil 
entirely in an air-dry environment, and under such conditions the ability 
to absorb water-vapor from the atmosphere and to quickly take up water 
when there are light rains would be a distinct advantage. 

It is still a question to what extent the rock-inhabiting lichens of the 
desert may be able to obtain moisture from the rocks on which they grow. 
The basalt is, in general, more dense and less porous than the tuff, and 
it seems likely that the latter may give off moisture so rapidly as to be- 
come too dry to support lichens very successfully through periods of 
prolonged drought. The less porous basalt doubtless gives off water- 
vapor very slowly, and this would be more favorable to the growth of 


SPALDING 


PLATE 12 





Lichens on north face of rocks above Desert Laboratory. 






eee | THe wBRARY, 
oie hte WNIVERSSTY OF HLLINGIS 


os > 
eo 4 Csuph 


ir . 
Tab 





PLANT ASSOCIATIONS AND HABITATS. aT 


lichens, especially on northward-facing exposures, where the effect of 
prolonged drought is least felt, and where, as a matter of fact, a very 
large percentage of the lichens occurring on Tumamoc Hill are found. 
The great number of lichens that almost cover the surface of small detached 
rocks on north slopes near the Laboratory seem to point to the atmos- 
phere rather than to the substratum as the source of water-supply, but 
the problem is worthy of much more thorough study than it has yet 
received. 





(10) CULTIVATED GROUNDS, WASTE PLACES, AND ROADSIDES; 
MISCELLANEOUS INTRODUCED SPECIES. 


Within the limits of Tumamoc Hill and the adjacent flood-plain the 
introduction of weeds and of various cultivated species has gone on until 
these have become, in many places, a conspicuous and more or less impor- 
tant element of the vegetation. The high mallow (Malva parviflora) and 
cocklebur (Xanthium canadense), for example, have greatly modified the 
aspect of roadsides and edges of fields. Along the arroyos Arundinaria, 
and near the old mill quince-bushes and Sapindus, are permanent re- 
minders of the planting of earlier days. Among later arrivals are the 
alfilaria (Erodium cicutarvum) and foxtail (Hordeum murinum), both of 
which have made themselves at home, and their presence is materially 
felt over wide areas. 

The physiological requirements of these and numerous other species 
introduced here, as well as their distribution in relation to local condi- 
tions, are, to all appearances, much the same as in the different regions 
from which they came, but it is noteworthy that they exhibit comparative 
indifference to intense insolation, much like indigenous species already 
referred to that commonly grow in shady places. With the one group 
as with the other, a sufficient supply of soil-water is evidently the first 
essential, and, with this assured, they flourish in the fierce glare of the 
sun in an atmosphere often of very low relative humidity. 











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CHAPTER Il. 


LOCAL DISTRIBUTION OF SPECIES. 


TOPOGRAPHIC AND SOIL RELATIONS. 


In the first division of this paper the aim was to present a clear account 
of the natural associations of plants on and near the Laboratory domain, 
and to point out, wherever possible, certain obvious relations between 
local distribution and environmental factors. In this division, dealing 
more in detail with constituent species of the associations, the attempt to 
trace cause and effect is carried a step farther. Certain species have 
been carefully mapped and their habits have been more thoroughly 
studied with reference to differences of soil and aspect. The species 
selected for special study are characteristic in their respective places, and 
the maps show at a glance their remarkable definiteness of habitat 
preference. 

The mapping has been done by Mr. J. C. Blumer, and the account which 
follows is based on his field-notes, which have been placed at my disposal. 
Observations of soil and air temperatures on opposite sides of the gulch 
near the Laboratory, made by myself for a number of months, aid in 
analyzing the complex factors determined by aspect, or direction of slope. 
The discussion of soil conditions by Dr. Livingston brings to light impor- 
tant relations and is presented in a later section as a special contribution. 

Beginning with the species that have been mapped, a certain amount 
of detail which is indispensable will be presented. 


ENCELIA FARINOSA. 


On the Laboratory domain, and in its vicinity, the lower limit of this 
plant coincides closely with the 2,500-foot contour, but its local distri- 
bution is evidently determined by other factors than altitude. It is 
usually limited to steep slopes, and its lower limit is often sharply marked 
by a few degrees difference in gradient. But steep slopes are here char- 
acterized by a thin layer of residual adobe soil of high retentive power 
-for water, and such slopes, with thin layers of soil, point to rock-masses 
near the surface, either in the form of fissured or laminated bed-rock, 
or of detached bowlders, conditions which indicate the presence of a 
relatively high degree of moisture. On this point see Livingston (1906). 
Thus, the rain of January 2-3, 1908 (0.62 in.), penetrated down about 
5 inches at the Laboratory, but only 3 inches at the farm near by. This 
indicates greater penetrability of rocky soil of this character. 

This species also shows in its distribution a marked and definite rela- 


tion to aspect. Other things being equal, its density increases directly 
29 


30 DISTRIBUTION AND MOVEMEN’S OF DESERT PLANTS. 


as the aspect becomes more southerly. But south aspect involves greater 
heat, and here, too, a steep slope also results in greater heat, at least 
during parts of the year, because of the more nearly perpendicular inci- 
dence of the sun’s rays. The accompanying map (plate 13) shows the 
almost entire absence of Encelia from north slopes and from the gentle 
slopes below the 2,500-foot contour. 

It appears, then, that FEncelia farinosa is limited to areas having a 
relatively high degree of heat and moisture. It grows more densely than 
any other large plant of the Laboratory domain, and is well limited 
to its own particular habitat. Within this habitat it is limited (a) by 
sheer cliffs; (6) by such an increase of rocks as to amount to bare talus; 
(c) by certain areas of creosote-bush, which may possibly prove signifi- 
cant as denoting an unusually great distance between surface and water. 


LARREA TRIDENTATA. 


The creosote-bush is the most abundant, and with the exception of 
Parkinsoma microphylla, and possibly Acacia constricta, the most widely 
and continuously distributed ligneous plant of the Laboratory domain. 
At the same time it is no less limited than Encelia, as regards its best 
habitat, by certain physiographic features. It occupies, either exclu- 
sively or as the major element, the gentle lower slope between the moun- 
tains and the flood-plain. 

Its upper limit, as a major element, is often very definitely marked 
by the same few degrees of difference in gradient as the lower one of 
Encelia, hence the two come together on the contour in such places (com- 
pare plates 13 and 14). Its lower limit, along the valley, is marked with 
absolute definiteness by the edge of the flood-plain, and accordingly 
its principal habitat is seen to coincide with the areas of coarse, gravelly, 
whitish, transported soil, devoid of large fragments of rock, which char- 
acterize the lower slope as already defined. 

Outside of its main belt the Larrea comes in strong in certain places of 
gentle slope and deep soil, places that have evidently been filled up in the 
past, but at present are wearing down. Some of these places are situated 
in the heart of the Encelia belt, but Encelia is absent. This confirms 
what the main belts show, namely, that Encelia and Larrea are of dis- 
tinctly opposite tendencies in their relation to certain determining factors 
of distribution. Encelia, as we have seen, follows areas having a relatively 
high degree of heat and moisture. Larrea, on the other hand, is preemi- 
nently a plant of well-drained erosion areas. ‘The main Larrea belt 
between the hills and the flood-plain is constantly wearing down, as is 
seen, among other things, from the numerous small gullies over this 
area, and the exposed sahuaro roots on the plains beyond the wash, where 
there is a strong growth of Larrea, tell the same story; even the bottom 
of the wash, where Larrea is present in large numbers, is at present wearing 










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SHADING iNDiCATES 


OF 





RELATIVE DENSITY OF THE SPECIES 


DIFFERENCE IN DENSITY 





NOTE: 





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MAP OF DESERT LABORATORY DOMAIN, SHOWING DISTRIBUTION OF ENCELIA FARINOSA 





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A HCEN & CO. BALTIMORE 


PLATE 4: 
; meat 
per too sq. meters 
Average of less than io and more 
than 1 per 100 sq. meters 
Widely scattered clumps | 
ST MARY'S \ 
HOSPITAL | 
None 7 
= ! 
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=x i 
12) 
x 
2 
TUCSON lea ¢ a4) 
COURT Mouse \\ MA | 
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Reger | A 
Mintamo | 
| 
L 
: pee pa Pere ae : | ¥ =. en ie J.C.Blumer del. 
MAP OF DESERT LABORATORY DOMAIN, SHOWING DISTRIBUTION OF LARREA TRIDENTATA OVER AN AREA OF ABOUT 1,800 ACRES 
SCALE: 315 INCHES PER MILE 


LOCAL DISTRIBUTION OF SPECIES. ot 


between the bushes, or at least not filling up. Where filling-up occurs, 
other plants come in. On the flood-plain, a filled-up area, Larrea is 
replaced by mesquite, and on the residual rocky slopes, where there is 
no abrasion going on, it is largely replaced by other plants. Compared 
with the mesquite, which is preeminently a plant of deposition areas, 
the creosote-bush is distinctly one of erosion areas. 

From extended observation, not only here but elsewhere in Arizona, 
the distribution of the creosote-bush may be said to present the following 
essential facts: 

(1) Larrea is largely a plant of transported as against residual soil; 
it is preeminently a plant of the gentle undulating slopes that lie at the 
base of the mountains throughout the southwest, giving out on the steeper 
slopes. above. 

(2) In contrast with Prosopis, a plant of the flood-plains, periodically 
subject to overflow and deposition, Larrea stands as the distinctively 
character plant of the adjoining erosion areas. Probably parallel is the 
fact that while the mesquite grows on sand, silt, and fine adobe, con- 
taining no caliche but relatively much humus, the creosote-bush grows 
in greatest numbers on dry, more or less gravelly soil, with little humus, 
and often a hardpan of caliche. 

(3) As compared with plants of the hill, Larrea is further a plant of 
deep as against shallow soil, expressing in a rough way proportional 
distance of bed-rock from the surface. It is also a plant of unstable talus 
areas, as long as there is soil present, and finally, it is a plant of the rela- 
tively highly stable mesas, which are nevertheless slowly wearing down, 
on the tops of hills and ridges. 

(4) As to certain observed relations to other plants, it frequently 
occurs conspicuously with Festuca octoflora, but loses its hold rapidly 
on areas dominated by [ranseria deltotdea, and over wide areas, where 
both may occupy ground of their choice, it is roughly inversely propor- 
tional to Encelza in its distribution. 

(5) It is not limited by altitude, except within very wide limits, nor 
is it manifestly affected by aspect. Thus the local distribution of Larrea 
presents a complex problem, upon which at present most light is gained 
by consideration of the sharp contrast it presents to various other species, 
notably Prosopis and Encelza, in its requirements. 


CEREUS GIGANTEUS. 


From the map (plate 15) it is at once evident that the sahuaro decidedly 
prefers southerly aspects; if other conditions could be eliminated it would 
increase in density directly as aspect grows more southerly. It is also 
seen, by reference to the map, that within limits the density of this species 
increases with the narrowing of intervals between the contour lines, that 
is, with increasing steepness of slope. In this, however, the substratum 


o2 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. - 


inevitably plays a large part, and it is hard to say how much is to be attrib- 
uted to this and how much to the degree of insolation. 

A third evident feature is the great increase in general density on the 
middle slopes of Tumamoc Hill, that is, the zone immediately below the 
talus belt; and it shows a decided preference for the finer soil that fills 
the interstices between rocks. This is darker, as a rule, has a smaller 
proportion of calcareous matter, is more retentive of water, and the caliche 
proper lies deeper, or is less in evidence. How much this manifest pref- 
erence is due to the presence of soil of this character and how much to the 
presence of rocks is uncertain, since the two almost always go together. 

The sahuaro differs in the evenness of its distribution locally, often 
forming groups where the topography and substratum are favorable. 
The following may be distinguished on the Laboratory domain: 

(1) Cliff groups occur on the sheer basaltic cliffs where aspect is right 
and are usually composed of smaller plants. Conditions for reproduction 
are good in many rock crevices, but, evidently owing to lack of soil, few 
plants reach any size. These plants are above the degree of steepness 
at which maximum density occurs. 

(2) Platy basalt groups occur where this variety of basalt is exposed. 
The most notable example is just under the southern brow of Tumamoc 
Hill, where a group of 22 is founded on this almost laminated basalt. A 
number of the smaller plants are giving way, indicating that reproduc- 
tive conditions are better than those for maintenance. 

(3) Scorvaceous basalt groups.—A few of these are found just north of 
the first quarry. In one 5 plants stand shoulder to shoulder, covering 
a tiny island of this basalt, while a very few scattering ones occur on the 
adjoining practically rockless shoulder. 

(4) Other rock outcrop groups.—Other kinds of outcrop give rise to 
sahuaro groups. Many good examples are seen on the south side of 
Sentinel Hill and the islands west of the wash. In fact, the sahuaro grows 
on all the kinds of basalt distinguished, as well as on the tuff. ‘The latter 
is well seen on the south side of Sentinel Hill. , 

(5) Talus groups occur about the lower edge of the talus, and are very 
significant as regards moisture supply. Borders of talus slopes are com- 
monly seen in the mountains supporting mesophytic species in the midst 
of a hot, southerly slope. They include here: 

(a) Terrace, or upper coarse talus groups: These are found mainly on 
the south side of the Laboratory domain, which is here the real sahuaro 
country. The groups occur usually on the side of the terrace, affording 
greater steepness and a rockier root-bed than the surrounding area. 
Usually the rock consists of bowlders embedded in the fine soil; some- 
times bed-rock terraces occur, as in the region of the copper-bearing 
basalt. 


e 


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PLATE 25 








Spe ® 
HOSPITAL on] 
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MAP 


SHOWING DISTRIBUTION 


OF 


CEREUS GIGANTEUS 









































es / 
er ~ ON THE 
os | 
Vas Sd ie 7 ci aa => ee ag) as Tr 4 mM T : 1 1 Tt \ r y | 
| TUMAMOC HILLS AND VICINITY | 
{ | | 
—— \ 
‘ TUCSON, ARIZONA | 
— | 1 | | 
= \ SCALE: 6000 he 
\ | | 
|- | | 
\ | 
| \ | 
| | 
| | 
; set } | 
: : ‘ \ : ane ia 
ae) aes = by oe Pee NT 2x Pd \ SS 
| y= Basgeiarogy’ i) f ay wes 
e Me = ina y ONG 
= = y = af | w EB & is (g | 
. a BA \ Bas i LEGEND 
ae 7. S 
me : Ay | Cereus giganteus 
/ \ # t One dot for each plant 
*% ft => 4° 
\ 8 gy , nes ae \2 \ / | Upper talus 
> ‘ as im Po. 
z. / = NA | | 
| + ei yA ’ > ~ — | / = E 
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[. | | 
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aN . / 
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| 
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| : sh a a a oS € = 7+ = ste {-—~ es rene - a = - 
| 
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| 
4 y 
, Ss 
— = a F ’ C. Blumer del,, April, 1go8 
yo 2 + 2 5 : 6 : 8 9 il i3 14 


LOCAL DISTRIBUTION OF SPECIES. 33 


(b) Detached lower talus groups, where coarse angular fragments are 
accumulated by the washing out of the fine material, are found near the 
foot of the slopes, dotting areas thickly strewn with basalt fragments 
imbedded in dark, fine, retentive soil. They usually lie in the bottom 
of depressions or along the lower course of arroyos. ‘This material has 
been brought down from above in times past. Some of these areas are 
good reservoirs for the storage of flood-waters. 

(6) Gully groups border the arroyos or gullies, and a collection of such 
groups forms a belt of unusual density following a dry watercourse, or 
a system of washes. Even where such a course is merely a band of sand 
it often has the power to attract to its sides the only plants of the neigh- 
borhood. Such arroyos often have the steep rocky banks which this plant 
affects, and it is quite probable that the roots also profit by the periodic 
flow. 

(7) Deltoid groups are probably the most significant of all. The smaller 
gullies on the slopes are not always continuous, but sometimes lose them- 
selves on a pile of rock débris deposited by floods. On the south slope, 
where they occur, their sides give a warm, southerly declivity, and the 
bowlder-filled soil is thoroughly saturated by every generous flood that 
comes along, while it receives the full value of even a tiny rill. All these 
habitats point to a high degree of warmth as essential, coupled with the 
best moisture conditions obtainable. 

The sahuaro is absent from the middle portions of talus slopes, and, 
speaking generally of mature plants, from cliffs, evidently because of 
lack of soil. It is comparatively scarce on northerly slopes, and present 
in intermediate numbers on east and west slopes. It is absent, either 
absolutely or comparatively speaking, from level land and from slopes 
cemented by caliche. 

The outline of the sahuaro groups is often determined by the physi- 
ography. ‘The belts of unusual density following arroyos and gullies are 
very evident, almost general, and follow the direction of these. Certain 
well-marked cases occur in depressions, where no water-channel is in 
evidence, but where the grouping takes the form of a belt running down 
hill, clearly marking seepage channels, either present or past, according 
to the general age and grouping of the plants. On the other hand, the 
long axis of certain other groups lies along the contour. ‘These will be 
found to grow along the sides of more or less horizontal terraces. A 
succession of such terrace groups, but, more generally, irregular rock- 
outcrop groups, may be seen to make a horizontal belt of unusual density, 
running about certain hillsides, notably Sentinel Hill on the south, clearly 
traceable to geologic strata of great comparative resistance to erosion. 
Protruding tuff-beds were found to be as prolific as outcrops of the different 
kinds of basalt. 


34 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


Another class of instances that may perhaps be put with the phenomena 
of dynamic physiography are presented by the slopes covered with loose, 
sliding tuff fragments about the three quarries, and notably on the south 
side of the west end of Sentinel Ridge. Only Larrea is able to maintain 
a more or less successful foothold, and even this occurs in distinct rows 
or beds running up and down hill, unable to resist the products of erosion 
sliding down the paths between. There is present on such slopes plenty 
of good-sized rock-fragments, as well as soil. ‘The aspect and gradient 
are the most favorable ones to be found, but the sahuaro in the case of 
the west end of Sentinel Ridge is almost altogether absent. Apparently 
the only thing lacking is a sufficiently stable surface, though moisture 
and other physical conditions may also be changed profoundly by this 
instability of surface. 

Associated with the optimum physical habitat for the sahuaro are often 
found certain recurring organic associations. ‘These are a rather unusual 
presence of species of Abutilon, Janusia, Lycium, and several others, on 
warm southerly aspects, in company with sahuaro groups. One of the 
best indicators of such an optimum habitat is Andropogon contortus. It 
almost certainly requires a high degree both of warmth and moistute. 
Hilarva cenchroides has been located on an area on the south side of Tu- 
mamoc Hill in company with a large group of sahuaros. A rare colony 
of Prosopis has been found on the same spot with a typical terrace group. 
Unusually large specimens of Parkinsonia mucrophylla, growing close 
together, are very commonly found associated with good sahuaro groups, 
occupying the same soil. Phoradendron californicum, on the whole 
rather infrequent, may with great certainty be found suspended, often 
in large thrifty pendents, from well-nourished specimens of Parkinsonia, 
standing among thrifty columns of the giant cactus. Not infrequently, 
where these occur at all, young giants may be looked for with success 
under the protection of both mistletoe and palo verde. 

The fact is to be emphasized that rocks, of whatever kind, as found 
on the Laboratory domain, are almost inseparably linked with the pres- 
ence of the giant cactus. These may carry with them the ultimate factors 
governing local distribution in the form of certain soil properties more 
or less constantly associated with the rocks, or the rocks themselves may 
act as water-conductors and water-reservoirs by means of the air-space 
enveloping them in dry times. The moisture of spaces between adjoining 
rocks, especially basaltic, may be heated by their presence, thus giving 
a temperature favorable for germination, as is suggested by the occurrence 
of the smallest sahuaros found growing in such places. The absence 
of the species appears to be caused by rapid erosion and instability of 
surface in some places, in others a prominent réle may be played by the 
impervious caliche, while in still others the most favorable conditions 
of rock and soil can not make up for the absence of a sufficient degree 
of insolation, or, what is self-evident, for the lack of seed. 





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Per eS WN 








LOCAL DISTRIBUTION OF SPECIES. 3D 


CERCIDIUM TORREYANUM. 


Before taking up this species, it is necessary to describe certain physio- 
graphic or soil areas which are referred to in connection with its local 
distribution. 


The west part of the Laboratory domain is divided naturally into four 
physiographic, or soil areas as follows: 


(1) The area in the northwest part of the Laboratory domain, thence 
extending some distance north and west, composed of gentle ridges or 
swells, with a definite substratum of caliche. It is the outer margin of 
the Tucson slope. Larrea is the characteristic plant and, on the drier 
portions, Redellia coopert. 

(2) The second area lies immediately south of the first, and extends 
from the main wash at the west base of Tumamoc Hill westward beyond 
the Laboratory domain to the foothills of the Tucson Mountains, certain 
ones of which, composed of rhyolite, are probably its sources. Its flat 
surface, with abrupt gravel bluffs along its washes, is characterized by 
coarse, reddish soil containing a large proportion of hard angular frag- 
ments. Caliche is present, but lies deeper than in the first area. This 
second area is marked by the presence in large numbers of Franseria 
deltordea and Opuntia fulgida, as well as the absence, over considerable 
stretches, of everything else except Parkinsonia microphylla. 

(3) The third area occupies the southwest part of the Laboratory 
domain, bounding the foregoing area on the south and extending along 
the main wash southwestward and southward to the wash which enters 
the Santa Cruz just south of Sentinel Hill. Its nearly flat or gently 
undulating surface is composed for the most part of fine, silty, and some- 
what sandy soil with caliche absent near the surface. The soil has appar- 
ently been deposited within comparatively recent time by the main 
wash. Though less homogeneous in its soil and plant cover, and possibly 
in its moisture supply, than the two preceding areas, it is nevertheless 
fairly well marked by distinctive physiographic features. The most char- 
acteristic plants are Ephedra trifurca and mesquite. 

(4) The fourth area includes the practically level bottom of the main 
wash, beginning at the point where this is deflected northward by Tumamoc 
Hill, and extending to the vicinity of St. Mary’s Hospital. Caliche is 
absent, except on areas apparently long abandoned by the water on its 
way toward the river. Wash species proper, as Cercidium torreyanum, 
Prosopis velutina, and Acacia greggu, dominate here. 


Within the Laboratory domain the boundaries of all four areas are 
for the most part very definite; they are indicated on the map (plate 
16) by broken lines. Small areas marked B are basaltic outcrops with 
a very distinct soil and might be designated collectively as a fifth physio- 
graphic area. The part of the map marked West Slope Tumamoc Hill 
represents an area of several different kinds of rock and soil and does 
not belong within the areas above described. 

Cercidium torreyanum is almost entirely confined to the washes; only 
in the second physiographic area does it occur side by side in the same 


36 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. - 


habitat with Parkinsonia microphylla, namely, the sides of the gravelly 
bluffs skirting the washes here. In the third physiographic area, also, 
a few instances are found where it occurs away from the washes, but 
probably these may be traced to the courses of former streams, which 
may be the courses of present underflow, or to places of comparatively 
extensive dissipation of currents within recent time. 

This species is much more abundant in the second physiographic area 
than the first, and the streams and streamlets are followed much farther 
up. It is also more numerously represented in the second than in the 
third area, but is most abundant in the fourth, or the wash bottom. By 
far the greater number of individuals stand in immediate proximity 
to the washes, where the periodic flow touches, or almost touches, the 
stem and undoubtedly reaches the roots. Of these a large number, pos- 
sibly a majority, stand either on the point of the prong formed by the 
junction or division of two washes, or on either bank immediately opposite 
such junctions or divisions. These places indicate the most copiously 
watered soil. 

By reference to the map (plate 17), it will be seen that along the main 
wash congestion, so to speak, takes place either at the principal bends 
or where the bed of the wash widens or divides and subdivides into numer- 
ous channels. Where the channels are comparatively narrow, straight, 
and without forks, the number of trees is comparatively small. ‘This 
and the foregoing paragraph indicate that this tree likes to have, it seems 
almost must have, its roots immersed in the periodical floods at points 
where the rapidity of their current is checked and their volume partly 
dissipated in the adjoining soil. 

At some points in the vicinity of Tucson, especially to the southward, 
on the road to the Sierritas, Cercidiwm torreyanum occurs on higher ground, 
and at higher altitudes to the eastward it takes its place with the general 
shrubby growth of the mountain slopes; but nothing has as yet been 
observed conflicting with the general fact that at this place it belongs 
to areas of transported soil, especially washes such as those of the Labo- 
ratory domain, where periodically the water-supply is very great, and 
probably never falls to the minimum of closely adjacent areas. An abun- 
dant water-supply and effective drainage are apparently the local factors 
determining its distribution. 


PROSOPIS VELUTINA. 


As already stated, the mesquite stands in sharp contrast to the creosote- 
bush in regard to distribution, the latter being distinctively a plant of 
erosion areas and the former of areas of deposition. There are occasional 
exceptions, or apparent exceptions, to this, as for example where, south of 
Tucson, a broad mesquite belt occupies a long, gentle slope from the 
Santa Ritas, joining the forest of the Santa Cruz flood-plain in a series 


PLATE? 

















| 





“MM ih 





«so 





ANH MA 


W 





\\t jain 





nea 





OH NN 


40 ACRES 


i 





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anit 
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hiiyy 
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————————— ee eS ere ‘ft gan 
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amaime FO ws / c 























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40 ACRES 
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LEGEND 








MAP 


ad Wash 


| Bro 


= 
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oa 
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OF 














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J. C. Blumer del. 








Phe Pe A otal awe « - = » 


a f . ‘ 


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Swe Bare ntg Mirko Mayra. 


¥ 
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Pte 





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= 
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a 
ca 


PLATES 





clumps, very widely 


Mesquite Woods 
scattered or rare 


| No Mesquite 


small 


=<- 


_, Small plants growing singly or in 


a 


LEGEND 








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) 


ST.MARYS 
HOSPITAL C 














rae | 


A. HOEN 4 CO. BALTIMORE 





CONGRESS ST. 





JUC.Blumer del 








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DESERT LABORATORY DOMAIN AND VICINITY, SHOWING APPROXIMATE DISTRIBUTION OF PROSOPIS VELUTINA OVER AN AREA 


3s INCHES PER MILE 


SCALE 


OF ABOUT 1,800 ACRES. 


LOCAL DISTRIBUTION OF SPECIES. 37 


of small ridges and troughs, a topography usually typical of Larrea. 
But such instances serve to direct attention to the fact that underlying 
the differences of topographical features there must be one or more con- 
trolling physical factors to which the widely different distribution of 
these plants is primarily due, and the first of these is apparently the 
amount and location of soil-water. 

A large number of observations, too numerous to be given in detail, 
go to indicate that the presence or absence of caliche has, locally at least, 
an influence on the distribution of these two species. All about the 
Laboratory domain the most exclusively Larrea areas are those where 
caliche comes most nearly to the surface in its characteristic layer, while 
the mesquite is, as a rule, absent from such areas. Nevertheless a com- 
parison of the map of Prosopis with that of Larrea (plates 14 and 18) 
shows that over considerable areas where neither is abundant the two 
may grow together or in close proximity. All this is consistent, however, 
with the principle stated above, since differences in amount of available 
soil-water are certain to result from the presence of the caliche, as it occurs 
at the base of Tumamoc Hill, and from the disposition of rocks and soil 
elsewhere. 


COMPARATIVE STUDIES. 


It is obvious from the foregoing that soil conditions, even within the 
narrow limits of the Laboratory domain, exercise a marked influence on 
the local distribution of plants, and this becomes still more evident when 
the plants of a wider area, with more pronounced differences of soils, are 
considered. 

For the sake of wider comparison in this direction, a study of the vege- 
tation of a number of areas in Arizona and New Mexico has been under- 
taken. That part of the Gila Valley lying between Solomonsville and 
Fort Thomas was visited by the writer in November, 1906. The work 
there was carried on with particular reference to ascertaining how far 
the plant habitats of the valley and their most characteristic plant species 
show a definite correspondence in their distribution with that of the 
various soils as they have been mapped by Lapham and Neill (1904), of 
the Bureau of Soils, U. S. Department of Agriculture. As shown by 
the map referred to (plate 19), a section of the Gila Valley at some place, 
Pima for example, where the different soils are well represented, crosses 
successively between the river bed and the lower mountain slopes on 
either side: (1) The Pecos sand, uniformly present on either side of the 
river in the valley trough; (2) the Gila fine sandy loam; (3) the Maricopa 
silt loam, a soil of very fine texture, with some of the peculiarities of 
adobe; these three constituting the alluvial soils of the district in dis- 
tinction from the three following, sometimes distinguished as colluvial 
soils, which are derived from the products of erosion of the mountain sides, 


PEAT Eso: 






SOIL MAP 


OF A 
PART OF SOLOMONSVILLE SHEET 


ARIZONA 












Scale ; 
1 2miles 








Maricopa 


sandy loam 





Maricopa 
sand 


Maricopa 
éravelly loam 







Soils surveyed by 
; Macy H Lapham and N.P. Neill 
Maricopa Riverwash 1903. 
silt loam 








LOCAL DISTRIBUTION OF SPECIES. 39 


(3) Passing to the Gila fine sandy loam, which comes next to the river 
sands, it is seen at once that its dense growth of well-developed plants has 
not suffered, for a long time at any rate, from the effects of periodical 
floods. The willows and cottonwoods are good-sized trees, 20 to 30 feet 
in height, the arrow-weed forms dense thickets, and Sueda moquini 
occurs, in places growing to such a size as to form woody stems some 2 
inches in diameter. As here observed, the vegetation of the Gila fine 
sandy loam may be characterized, first, by the presence of most of the 
species of the Pecos sand in a much more advanced stage of development; 
second, by the presence of other plants adapted to its peculiarities of 
texture and drainage, but incapable of successful resistance to floods; 
and third, by the absence of various species that affect the heavier soils 
beyond. 

(4) The Maricopa silt loam and Maricopa sandy loam, which grade 
into each other, constitute the distinctive habitat of a dense mesquite 
forest, which upwards of 30 years ago covered a large part of the valley 
and was grubbed out by the Mormon settlers. All the best of this land 
is now divided into productive farms, but enough of the native vegeta- 
tion remains to make it plain that in earlier years there were, as now, 
two well-marked associations of plants on this area, which practically 
corresponds with the flood-plain of the river. These are essentially 
identical with the two associations of the flood-plain of the Santa Cruz 
River, already described, namely, the mesquite forest association, con- 
sisting of a thick growth of mesquite, catclaw, and a few shrubs, with a 
lower growth of Atriplex canescens and a fairly numerous complement 
of herbaceous perennials and annuals; and the association of the salt- 
bushes, here exceedingly well represented and covering large areas, miles 
in extent, on which at the time of observation Atriplex nuttallu and 
Sueda moquint were the almost exclusive occupants of the soil. Even 
these species fail on some of the worst spots marked on the alkali map 
of Lapham and Neill (J. c., p. 24). On one adjacent to the Fort Thomas 
Canal there was a good deal of bare ground and much of the Sweda was 
dying. 

No more perfect correspondence of soil and association could well be 
found than exists here, the mesquite forest occupying the flood-plain, 
with its fine silt or sandy loam and a sufficient supply of water, and the 
Atriplex and Sueda forming the association of the salt-spots where the 
percentage of alkali is too high for most other plants. 

(5) The Maricopa sand, which succeeds the preceding soils as we ap- 
proach the lower slopes of the mountains, is characteristic in texture 
and topographical position, and also presents a characteristic vegeta- 
tion. Here the creosote-bush begins, Yucca elata and certain cacti make 
their appearance, and Afriplex polycarpa attains its best development. 


40 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


Guterrezia euthamie is also frequent. ‘The replacement of Prosopis by 
Larrea and of Atriplex nuttallit by Atriplex polycarpa, the coming in of 
cacti and the appearance of Yucca and Gutverrezia, are some of the most 
distinctive marks of the vegetation of the Maricopa sand as here observed. 

(6) The Maricopa gravelly loam, forming the slopes and terraces above 
the Maricopa sand, is distinctively the home of the creosote-bush and a 
larger number of cacti. With these were found at Pima and near Solo- 
monsville Kerberlinia spinosa, Yucca elata, Acacia constricta, and some 
others, and on the flat terraces Fouquierta splendens. The habits of 
Guherrezia euthanuie, which also occurs here, are the reverse of those of 
Fouquieria as to local choice of habitat. It is hardly seen except on the 
slopes, which at Solomonsville were yellow with it for miles, while on 
reaching the top of the terrace hardly a specimen could be found. 

Some of the species that have been named are far more restricted as 
regards limitation to a special habitat than others are. As examples of 
those more closely restricted in their choice may be named Atriplex 
polycarpa, which belongs definitely to the Maricopa sand and is scarcely 
found beyond it, and Atriplex nuttall, which grows altogether upon the 
Maricopa silt loam, or at most extends into the Maricopa sandy loam. 
The creosote-bush is limited, almost strictly, to the Maricopa gravelly 
loam and the Maricopa sand, ocotillo to the Maricopa gravelly loam, 
and the various species of cacti to this and to the Maricopa sand. 

The mesquite, on the other hand, although attaining its best develop- 
ment on the Maricopa loam, ranges from the Gila fine sandy loam to the 
gravelly loam of the mountain slopes, and Aérzplex canescens has an even 
wider range, since it occurs also on the Pecos sand. A number of species 
grow on any soil whatever, from the shifting river-sand to the heaviest 
silt loam, apparently with almost no limitations, even as regards the 
amount of alkali in the soil. Arrow-weed (Pluchea sericea) is one of 
these, and cocklebur and some other introduced species exhibit the same 
indifference to physical and chemical peculiarities of the soil. 

The details that have been given point to certain important conclusions: 

(1) It is evident, in the first place, that there is a remarkable cortre- 
spondence between topographic features as the result of physiographic 
processes and the local distribution of plants in the Gila Valley. The 
tiver-banks, the flood-plain, the long slopes approaching the mountains, 
and the steeper slopes and terraces above are severally the habitats of 
different and well-marked associations of plants. Here, as well as in 
the eastern United States, it may be said that physiography furnishes 
an efficient basis for an ecological classification of the vegetation. (Com- 
pare Jennings, 1908.) ? 

(2) But such studies as have been conducted here make it plain that 
soil conditions, differing greatly in the different physiographic areas, are 
the real determining factors governing the local distribution of plants. 


LOCAL DISTRIBUTION OF SPECIES. 41 


The evidence is such that we may unhesitatingly name soil-water content, 
percentage of alkali, and texture of the soil as three of the more efficient 
edaphic factors controlling the choice of habitat exhibited by the plants 
of the Gila Valley. The occurrence of the cottonwoods on the river- 
banks, of the mesquite on the flood-plain, and the creosote-bush on the 
lower mountain slopes is certainly conditioned by the first, and the case 
of the arrow-weed, which grows in any soil whatever where there is water 
enough, is still more striking. The constant association of Sueda moquini 
and Atriplex nuttalli1, almost alone over miles of salt-spots, both drained 
and undrained, is sufficient evidence of the efficient action of the second 
factor, and the behavior of certain other species points as clearly to a 
choice based on physical peculiarities of the soil. Thus Aériplex poly- 
carpa in its close adherence to the Maricopa sand suggests neither water- 
supply nor percentage of alkali as controlling factors, but more probably 
a demand on its part for more perfect aeration than is afforded by the 
heavier soils beyond. 

(3) The absence from the mountain slopes of the upper Gila Valley 
of plants so conspicuously present in similar situations on the Laboratory 
domain as the sahuaro, Encelia farinosa, and Parkinsonia microphylla 
points to the fact that general climatic conditions are also potent in 
determining what plants shall and what shall not hold their places here 
in associations of which they are elsewhere important constituents. From 
evidence obtained on Tumamoc Hill (p. 47), it appears probable that 
temperature is the determining factor in the cases referred to. The 
consideration of this phase of the subject is reserved for another place. 

(4) But whatever differences of characteristic constituents may appear 
in the same associations, as they are represented in the valleys of the 
Santa Cruz and the Gila, the capital fact remains that in the local dis- 
tribution of their plants these areas are essentially alike. We may add, as 
subsequent studies of the writer have shown, the Salt River Valley, so 
that what has been said applies generally to all the great valley systems 
of southern and southwestern Arizona. Beyond these valleys are the 
mountains with their mesophytic vegetation. It is in the valleys and 
on the adjacent slopes that the characteristic desert vegetation attains 
its best development and exhibits the peculiarities of local distribution 
that have been described, and since one great valley is fundamentally 
the counterpart of every other, a thorough study of one becomes the 
means of interpretation of all the rest. 

(5) In the Gila Valley, as in that of the Santa Cruz, and in general 
as far as these observations have extended, everything indicates that causes 
now in operation have determined the actual distribution of plants in 
the associations and habitats where they are now found. The general 
physiographic features remain constant, but boundaries are continually 
shifting, and coincidently with these changes the plant associations retreat 


42 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


slowly from old positions and advance into new ones, as apparently they 
have done from an indefinite past. If this view is correct, the study of 
present local distribution and movements is the surest guide to an under- 
standing of those of earlier times. 


Observations carried on in more widely separated regions, particularly 
in the desert near Alamogordo, New Mexico, and along the western limits 
of the Salton Basin, have confirmed the results that have been indicated 
as regards the importance of physiographic features and soil characters 
in determining local distribution of plants. As to the latter factor, how- 
ever, perhaps no more striking case can be found than that reported 
by Mr. J. C. Blumer in his recent studies of the ecology of the Fort Bayard 
(New Mexico) watershed,' in which, within a comparatively limited area, 
no less than seven distinct societies of plants, each referable to its special 
rock-formation, are clearly recognized and described. ‘Thus, giving them 
the name of the rock on which they occur, there are: (1) limestone, (2) 
quartzite, (3) granite, (4) andesite, (5) rhyolite, (6) conglomerate, and 
(7) basalt societies. Very remarkable differences of composition are 
noted in societies close together, or even adjoining. Of two adjacent 
areas, each including 0.05 acre, meeting on the contour line that marked 
the contact between basalt and conglomerate, that on the basaltic slope 
showed 41 individuals of Agave applanata and one of Quercus arizonica, 
while that on the conglomerate had 36 of Quercus artzonica and none at 
all of the Agave. 


Aspect in the same region is recognized as a powerful, and in fact the 
most powerful factor; and the analysis of the flora, based on this, presents 
very striking evidence as to its importance as a factor in local distribution. 
As an example, ‘the east side of Cameron Canyon changes repeatedly 
from west to northwest aspect, and on the latter not more than 30° in 
aspect is sufficient to give the entire slope, from top to bottom, over to 
bull pine.’’ Similar cases have been observed everywhere, but they 
are particularly well marked in the semi-arid regions of the southwestern 
United States. An extremely interesting locality in New Mexico was 
visited by the writer in July, 1906, where at an altitude of 6,000 feet 
the flora of the pifion association occupies the left bank of a narrow can- 
yon, while on the right bank, hardly a stone’s throw distant, are the 
plants of the desert below—mesquite, creosote-bush, ocotillo, Yucca, 
Agave, and various cacti. 


On the Laboratory domain, the floristic differences presented by neigh- 
boring areas, due to differences of aspect, though generally less striking 
than those just referred to, are sufficiently marked to afford a favorable 
opportunity for an investigation of the differences of environment to 





1Unpublished notes communicated to the writer. 


LOCAL DISTRIBUTION OF SPECIES. 43 


which they owe their origin. For this study the gulch located a few rods 
southwest of the Laboratory has been selected, because of its convenience 
of access and because other differences than those resulting from aspect 
are negligible. This gulch has been formed by erosion and is still slowly 
extending its area. By reference to the geological map it is seen that 
the underlying rocks are of the same character on both sides. Whatever 
differences of soil, therefore, are observable at the present time are not 
to be referred to original differences of the rocks, but are due to changes 
incident to its gradual formation, such, for example, as the accumulation 
of vegetable mold on the shady side. 

A comparison of the photographs of the two sides of the gulch Ai points 
directly opposite (plates 9 and 10) shows at a glance some of the more 
striking differences in their plant covering. The sahuaros, which are a 
most conspicuous feature on the right side, facing west of south, are 
hardly seen on the opposite side, and the same is true of Encelza farinosa. 
On the other hand, there are various plants on the shady (left) side which 
are either very meagerly represented on the right or are not found there 
at all. 

For the purpose of more exact floristic comparison of the two slopes, 
six areas of 100 square meters each were located, three on each side of 
the gulch and as nearly opposite as practicable, and a careful enumera- 
tion made of the perennial species of plants growing upon them. Peren- 
nials alone were counted, for the reason that they constitute the more 
permanent vegetation, though the annuals present the same general 
facts of distribution. The use of such marked areas gives a fairer com- 
parison than would a collection at large, though the latter would increase 
the number of recorded species. On these six areas the whole number 
of perennial species found on both sides of the gulch was 48, of which 
24 were found on the right side and 39 on the left. The number of species 
within these areas common to both sides is 15. There were 9 species 
found on the right side which were not found on the left, and 24 on the 
left side not found on the right. 

The most significant fact is that upon these representative areas more 
than two and a half times as many species of perennials were discovered 
growing exclusively on the left bank with its northerly exposure than 
on the right bank of the gulch with its exposure to more severe desert 
conditions. 

Still more impressive is the great difference in number of individual 
plants. As already stated, certain species well represented here show 
no aspect preference; but with many the case is widely different, as is 
shown by the list of 10 characteristic species on the six marked areas. 
(See table 1 on page 44.) 


44 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


TABLE 1.—Characteristic species on the marked areas. 





No. of indi- No. of indi- 









































: viduals. . viduals. 
Species. Species. 

Right.) Left. Right.| Left. 
Encelia farindsa,.. ee eee | 1r2 3 Spheralcea pedata....... 4 a7 
Cereus giganteus sae. 45 eS) O Brickellia-‘coultera.. 22 & « 4 28 
Menodora scabra 2 aa 12 94 Lippia wright see yas O 2 
ADUtUON CHSp Mie ye eee 14 O Brodiza capitata......... O IIO 
A Mcanuniye. ee. ae eee oO 52 Janusia gracilis soe) .&.. > II 86 








A comparison of the annuals, especially as regards numbers of indi- 
viduals, would be still more striking. When the winter annuals are in 
their prime, those on the right bank make no impression on the eye from 
the opposite side, while by reversing one’s position and looking over on to 
the left side from a point near the Laboratory, its whole face appears 
covered with a rank growth of annuals. There is an apparent lack of 
good soil on the right side, but none the less there are many spots where 
there is good soil that is unoccupied, or almost unoccupied, by plants. 

This condition of affairs has evidently come about gradually. Humus 
has slowly accumulated on the left side of the gulch, and the conditions 
have improved from year to year for the growth of plants, but the original 
difference of the two sides is one of aspect, and it is this that is to be 
regarded as the primary cause of the great differences in their vegetation. 

We may, therefore, consider aspect, or direction of slope, from a purely 
physical point of view, as the essential cause from which, as a starting- 
point, the differences of condition and of vegetation already noted have 
taken the form which they exhibit to-day. Originally and fundamen- 
tally, the difference between the two sides of the gulch is simply one of 
insolation, a difference that may be expressed in physical measurements. 

It was thought worth while, therefore, to make a series of temperature 
records for a period of some months. ‘These were begun December 17, 
1906, and brought to an end May 20, 1907. Measurements of light inten- 
sity and readings of wet and dry bulb thermometers were also made 
during a part of this period, and reference will be made to these later. 

For the temperature records four sets of instruments were employed, 
namely, two registered maximum and minimum thermometers for soil 
temperatures; four ordinary soil thermometers; a number of black-bulb 
thermometers, and a set of standard U. S. Weather Bureau thermom- 
eters for air-temperatures, all of which were tested and compared at the 
beginning and during the progress of the work. ‘Two stations were 
established on opposite sides of the gulch at points where extreme con- 
ditions prevail; that on the right side being exposed to full sunlight 
nearly all day, while that on the left was backed by a wall of rocks and 
was well shaded. The maximum and minimum thermometers first men- 


~ 


LOCAI, DISTRIBUTION OF SPECIES. 45 


tioned were placed in galvanized-iron tubes and sunk in the earth so that 
the sensitive bulb was 1 foot below the surface of the ground, the space 
above it being filled with a plug of earth, also inclosed in a galvanized- 
iron tube, so as to secure readings representing as closely as possible the 
temperatures at that depth. They were also placed, during a portion of 
the period of observation, with the bulb an inch below the surface. The 
ordinary soil thermometers were placed so that the bulbs were 2 inches 
below the surface. Two of these were employed, one in full light and 
the other in shade, at each station. The black-bulb thermometers were 
placed with the bulb 1 to 2 inches above ground, and readings were taken 
in full sunlight and in shade at each station. The Weather Bureau 
maximum and minimum thermometers were set up for comparison at 
each station, each being artificially shaded. 

The readings taken during the period of observation are too volumi- 
nous for record here, but are given in part, on p. 98, in the section on 
climatic conditions. Only the more important results will be given in this 
place. 

(1) Minimum and maximum soil thermometers, bulb at depth of 1 foot: 

(a) During the period of observations in December the average mini- 
mum and maximum were, for station I, 54° and 57° F. and for station 
II, 49° and 49.5° F.; that is, the minimum at station I, averaged 5° and 
the maximum 7.5° higher than at station II.! 

(6) In May, during the period of observation, the average minimum 
and maximum for station I were 74.5° and 80° F., and for station II 
70.5° and 73° F.; the minimum at station I averaged 4° and the maxi- 
mum 7° F. higher than at station II. 

(c) The difference between minimum and maximum at station I was 
from 2° to 3.5° F. in December, and from 4° to 7° F. in May, while for 
station II the corresponding differences were 0.5° in December and 1° 
to 4° in May. 

(2) Minimum and maximum soil thermometers, bulb at depth of 1 
inch below the surface. The observations recorded in March, April, and 
May, table 7, with the bulb 1 inch below the surface, show, as would be 
expected, a far wider range of temperature. 

(a) The average minimum and maximum at station I were 62.5° and 
95° F., and at station II 60° and 70° F. 

(6) The difference between minimum and maximum readings at station 
-I ranged from 30° (March 29) to 40° F. (March 25). At station II the 
difference ranged from 6.5° (April 12) to 13° F. (March 29). 

(c) While the minimum at station I averaged only 2.5° higher than 
at station II, the maximum averaged 25° F. higher. 











1Slight apparent discrepancies in these averages are due to the fact that they are 
derived from a larger number of temperature records than are given in tables 8 to 10, 


p..98. 


46 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


These observations were supplemented by many others taken with 
the ordinary soil thermometers at a depth of 2 inches below the surface. 
Readings of these were made from January 25 to May 20, 1907. Within 
this period the lowest and highest readings at station I were 62° and 
119° F., and at station II 54° and 112° F., the lowest in each case being 
in the shade and the highest in the sun. It is to be observed that whether 
in sun or shade, the temperature of the soil at a depth of 2 inches was 
always higher at station I than at station II. The difference on some 
cloudy days was no more than 3°, but on sunshiny days a difference of 
10° to 15° F. was frequently noted, and in some cases as much as 20° 
COV25 0? 

Readings of the black-bulb thermometer were made from January 26 
to May 20, 1907, the temperature being taken at 1 to 2 inches above 
the surface, in nearly all cases in the middle of the afternoon, but part 
of those in April and May at about 10 a. m. 

From the table on page 98 it is seen that: 


(a) The temperature of the air at 1 or 2 inches above the surface, 
from January 26 to May 20, averaged about 3° higher at station I than 
at station II (readings in each case in the shade), but frequently the 
difference was as much as from 8° to 12° F, 

(6) Differences in readings at points in sun and in shade on either 
side of the gulch are nearly the same, running much of the time from 
January 26 to May 20, between 20° and 35° F., though coming up several 
times to 40° and in one instance to 42° F. 


Summarizing the observations of temperature for the two sides of the 
gulch, it appears that on the right side, with southerly exposure, the 
soil temperatures have a much wider range than on the opposite side 
and in the daytime are much higher, the average observed maximum 
for 3 months being some 22° F. higher than that of the left side at a 
depth of 1 inch. These differences become less at greater depths, but 
still at a depth of 1 foot the soil is from 4° to 7° F. warmer on the right 
than it is on the left side, the difference being greater in winter than in 
summer. The roots of plants growing on the right side of the gulch, 
except as they penetrate to considerable depths, are in a soil always 
warmer, and in the winter, in the daytime, very much warmer, than 
that of the left side, but they are also exposed to much greater extremes 
of temperature. 

The air temperatures (in the shade) are also higher by 5° or more on 
the right side than on the left (often much more), but the greatest dif- 
ferences of temperature on either side are those of full sunlight and shade, 
running from January to May for the most part between 20° and 40° F. 
As the left side is the shady and the right side is the sunny one, during 
all the cold months of the year, these differences are greatly in favor 
of plants sensitive to cold that grow on the right side. When the sun is 


LOCAL DISTRIBUTION OF SPECIES. 47 


high in summer the differences are less marked, but perhaps are no less 
significant as regards plants growing on the north exposure which are 
sensitive to extreme desert conditions. 

It is evident that, starting with temperature as the initial factor, semi- 
tropical desert conditions are distinctly emphasized on the right side 
of the gulch, and that these conditions are greatly mitigated on the left, 
and it can not be doubted that the great differences of their plant covering 
are directly correlated with these differences of temperature and the 
changed conditions to which they have given rise. 

As already stated, observations of light intensity and readings of wet 
and dry bulb thermometers were carried on for some time; but with the 
growing conviction that with our present methods no satisfactory results 
could be looked for, and with increasing evidence that, for such a compar- 
ison of habitats, light intensity at least, is a matter of very subordinate 
importance. This part of the work is accordingly not reported here. 


MEANS AND AGENCIES OF DISPERSAL. 


The various plant species of Tumamoc Hill are provided with means 
of seed dispersal essentially the same as those found in all great conti- 
nental areas of the globe. The pappus-bearing composites, which are 
fairly numerous, are adapted to wind dispersal; many others, including 
species of Boraginaceze, Umbelliferae, etc., have spiny or hooked fruits; 
others, such as Lycium, Celtts, and the cacti, have fleshy fruits likely 
to insure dispersal through the agency of birds and other animals, and 
to these are to be added various capsular fruits, grasses with twisted 
awns, and still other mechanical contrivances of various degrees of effi- 
ciency in local dissemination. 

Of the several agencies of dispersal, the strong winds of the desert rank 
high in efficiency, and there is no doubt that light, winged seeds are 
borne on the wind to great distances. Exact observations are, in the 
nature of the case, difficult to obtain, but the evidence is ample. Taking 
25 or 30 miles as a measure, far within the limits of safety, of the possible 
annual advance of a species with light seeds adapted to wind dispersal, 
it is certain that in a few years such species may have migrated hundreds 
of miles from their point of departure. Indeed, it can not be doubted 
that their geographical limits are imposed by environmental conditions 
rather than by means of dispersal. Given the time element, there is no 
-part of western America, in which suitable conditions prevail, which 
such species may not reach. Their present actual range may, therefore, 
be looked upon as a resultant of a number of different factors, the deter- 
mination of which presents a most intricate biological problem, involving 
determination of capacity for dissemination, often practically unlimited 
except by natural barriers, and the controlling influence of the most 
diverse natural environments. 


4S DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


Much the same, though with certain obvious limitations, may be said 
of the species that are disseminated by the agency of birds. The well- 
known fact that some birds migrate practically the whole length of the 
American continent, from beyond the Arctic Circle to Patagonia and 
return, indicates the possibilities in this direction. 

W. E. D. Scott has named 248 species of birds as occurring within 
the region extending 80 miles north and 40 miles south of Tucson, of 
which, aside from those ranging through the whole region, “common 
on the plains” or “among the giant cacti,’ and therefore to be counted 
in, no less than 69 are specifically mentioned as found in the vicinity 
of Tucson. (See Bailey, 1902.) 

When it is considered that this list includes ducks and other aquatic 
birds, doves, woodpeckers, jays, crows, and many other birds whose 
efficiency as distributors of seeds is well known, it is seen at once that 
the birds which visit Tumamoc Hill and the adjacent valleys are both 
numerous and active enough to count as a factor of prime importance 
as an agency of dissemination. Exact observations of species known 
to be introduced by their means are not easily made, but from what has 
been seen of the feeding on cactus fruits and mistletoe berries by wood- 
peckers, and the extent to which the seeds of various crucifers, plantains, 
and other annuals are eaten by various small birds common about the 
Desert Laboratory, it is perfectly certain that many species of plants 
now growing here are carried year after year, often to long distances, by 
birds, and presumably found their home here originally by the same 
‘agency. According to Tuomey, “nearly a half hundred birds feed upon 
the fruit of the giant cactus, the list including all our thrashers, wood- 
peckers, finches, and pigeons.”’ 

But while birds function most conspicuously as disseminators of seed 
to the advantage of the species thus distributed, their relation to the 
vegetation of Tumamoc Hill is, in some few cases at least, unequivocally 
destructive. Thus the Gila woodpecker, Melanerpes uropygialis, makes 
large holes in the giant cacti by which the natural protection of the soft 
parts within is so far destroyed that decay, often involving the destruc- 
tion of the plant, ensues. 

A considerable number of mammals, conspicuous among which are 
jack-rabbits, squirrels, rats, and gophers, make their home on Tumamoc 
Hill or in its vicinity, and are in close relation to the vegetation either 
as active agents of seed dissemination or as destroyers, some of them 
playing a double réle very effectively. Squirrels here, as elsewhere 
(compare Bailey, 1905), fatten on the fruits of the bisnaga (Echinocactus 
wislizent), and these and other rodents are doubtless responsible to a 
very large extent for the transfer of various edible fruits and seeds to 
limited distances, and very likely assist materially in carrying from one 
point to another seeds that are provided with hooks and similar appen- 


¢ 


LOCAL DISTRIBUTION OF SPECIES. 49 


dages. Asa destructive agency, the mammals of this region are respon- 
sible for losses which would be far greater were it not for the extraordinary 
degree of protection which so large a proportion of desert plants enjoy. 
The jack-rabbit, when pinched by hunger, attacks the flat opuntias of 
the hill and manages, in spite of thorns and spicules, to gnaw out large 
portions of joints, the plant presenting an unsightly appearance as the 
result, though never, as far as observed, entirely destroyed. 

The part played by rodents in the dissemination of seeds on the Labor- 
atory domain has probably been far greater than at first sight appears. 
Following the unusually heavy rains of the winter of 1904-05 the palo 
verde (Parkinsonia microphylla) on Tumamoc Hill bore an abundant 
crop of seeds, but on my return to Tucson in the fall it was with difficulty 
that a mere handful of pods could be found. The seeds are unprovided 
with any special means of dissemination, except as they contain food- 
substances, and there is no other explanation of their disappearance so 
probable as that they have been carried away for food by rats and squirrels, 
which are often very numerous in the vicinity of the Laboratory. In 
times of protracted drought, too, these animals are seen, emboldened by 
hunger, to carry away every vestige of possible food left on the ground. 
Following the dry winter of 1903-04 it was noticed that the sparse 
growth of annuals hardly appeared above ground before it was closely 
cropped; so that normal growth was quite impossible, owing to the 
presence of numerous half-starved rodents in search of food of any kind, 
which was eagerly snatched, even when they were watched at close range. 

At such times the destructive work of these animals becomes, it would 

seem, a factor of considerable magnitude in limiting the spread of certain 
species; but from their great abundance in following years of rainfall, 
it does not appear that the life of any species has been threatened. In 
short, the probability is that, with the exception of the giant cactus, 
which suffers, as already stated, from the work of wood-peckers, there 
are few, if any, species on the Laboratory domain that have been ser1- 
ously interfered with by animals. The case, of course, is different on 
the flood-plain of the river, where overpasturing has induced great and 
irreparable losses, but these changes belong to those induced by human 
agency and fall into a separate category. Allin all, the mutual relations 
of plants and the larger animals here are, at the present time, distinc- 
tively advantageous to each. 
As far as pertains to strictly local distribution, it is clear that ants, 
which are numerous both in species and individuals, are highly efficient 
agents. They are seen on all parts of the Laboratory domain, busily 
engaged in gathering seeds of various plants and carrying them to their 
quarters. Seeds of the following genera, thus carried by ants, have been 
identified: Microserts, Plantago, Lesquerella, Harpagonella, Cryptanthe, 
Daucus, Amsinckia, Festuca, Ervophyllum. 


50 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


The number of plants on the Laboratory domain that are provided 
with hooks, spines, etc., is relatively large. At certain seasons of the 
year it is almost impossible to walk about on Tumamoc Hill without 
carrying on one’s clothing a collection of burs of Harpagonella, the siliques 
of Thelypodium lastophyllum, or the equally tenacious fruits of other 
genera. ‘The seeds of such plants are distributed in great numbers by 
domestic animals, and doubtless by others whose part in transportation 
is less easily observed. The part played by domestic animals is perhaps 
most conspicuously shown by the ubiquitous cocklebur, which, from 
Texas through New Mexico and Arizona to California, has crowded into 
roadsides, river-bottoms, and waste lands generally, and is everywhere 
seen with its prickly burs matting the tails of horses and cattle, and 
constantly carried by them to whatever fresh spots have not yet been 
invaded. <A striking instance of the interrelations of different animals 
with a single plant, now widely distributed through the southwest, is 
described by Tower (1906). Solanum rostratum, which grows as a weed 
around corrals and watering-places, has followed the two chief routes 
of travel from northern Mexico into the United States—the one into 
Texas, the other into New Mexico and Arizona—having been carried 
along these routes in early times, as now seems certain, by Spanish pack- 
trains. This is the favorite food-plant of Leptinotarsa intermedia, a spe- 
cies closely allied to the Colorado potato-beetle. Apparently the beetle 
and the Solanum extended their range together from Mexico early in 
the eighteenth century, and to the present time have advanced parz 
' passu, since it is found at the present day that as new routes of travel 
into irrigated arid lands are opened they move into them and together 
occupy lands where they had not hitherto been found. Thus incident- 
ally, in the Spanish conquest of Arizona and New Mexico, the animals 
of the pack-trains brought with them and effectually disseminated the 
seeds of a plant, which by other means, probably by herds of bison, was 
later carried farther north, where it has still been the food of Leptin- 
otarsa intermedia, and for a time of the economically more important 
and dreaded allied form, the potato-beetle. 

Water is perhaps to be classed with the agencies of dissemination 
which, in the region of the Laboratory domain, are of minor importance, 
but it is none the less an efficient factor. The heavy rains that gulley 
out the mountain sides, though relatively infrequent, carry multitudes 
of seeds from higher to lower levels, the tendency being to obscure zonal 
distribution, in so far as the action of entirely different factors makes 
this possible. In some parts of the desert region, however, it is probable 
that at the present time water is the most effective of all the agencies 
named. During the Salton Sea expedition of February, 1908, Dr. D. T. 
MacDougal and Dr. W. A. Cannon visited various points of the shore 
and the islands in the southern part of the sea, for the purpose of ascer- 


LOCAL DISTRIBUTION OF SPECIES. ol 


taining what plants had become established on ground laid bare by the 
recession of the water. At this time the shore-line had receded about 
half a mile from where it was at the maximum high-water level of Feb- 
ruary 10 and 11, 1907. They found that in all cases new introductions 
to any strand appeared to have come by water, although taken from the 
parent plant by wind. A striking case was observed on Obsidian Island, 
on the side facing the inflowing currents from the Alamo and New Rivers 
and subject to the action of wind-driven waves, where, out of 24 species 
collected, no less than 19 appeared to have come from seeds deposited 
by water. At a point on the southwest shore of the sea, where a long 
gravel delta comes in from the westward, 8 species of plants were found 
growing on the emersed zone, 2 of which, judging by obtainable evi- 
dence, floated there on the water of the lake, while the occurrence of 
the remaining 6 was such as to indicate the transportation of their seeds 
by currents of small streams from the slope above. From these and 
similar observations made during this expedition, it appears that water, 
whether of the sea or of streams flowing into it, has thus far been the 
most important agent in transporting the seeds of plants that have become 
established about the shores of the Salton Sea since the recent recession 
of its waters began. 


INVASION, COMPETITION, AND SUCCESSION. 


The succession of certain societies of plants on Tumamoc Hill and in 
its vicinity is a matter of simple observation involving no special diffi- 
culties. Thus, we have the frequently cited case of the creosote-bush, 
which is everywhere seen as the dominant growth on alluvial fans and 
the long slopes, and which moves upward with the changes due to erosion 
and deposition. Equally plain is the advance of the palo verde-catclaw 
association, as the washes extend upward through the mesa-like slopes. 
In this case, although the succession is plainly marked, the displacement 
of the preceding association is partial, not complete, since Larrea, its most 
characteristic constituent, remains in the new association and evidently 
profits by the change of conditions. 

But however simple such general movements may appear, a very few 
years of continued observation are sufficient to establish the fact that 
minor changes are constantly taking place, and that by this means the 
invasion of new forms is gradually changing the composition of the vari- 
ous existing associations. For example, the extension of the gulch just 
to the southwest of the Laboratory has brought in its train, in place of 
the association characteristic of the gentle north slope of the hill, the 
establishment of two remarkably different ecological groups on its oppo- 
site sides, which have already been described. 

Of much interest, though perhaps of less permanent influence on the 
character of the vegetation, is the invasion of certain species of annuals 


52 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


which have offered a favorable opportunity for continued observation. 
Among the species that have been carefully observed, Evodiwm cicutarium, 
or alfilaria, is of such exceptional interest as to deserve detailed descrip- 
tion. This plant is a native of the Mediterranean region and was intro- 
duced into the New World by the early Spanish explorers. It was 
introduced into Arizona from California, according to Thornber (1906), 
as early as the sixties. At this period numerous freighters brought over 
hay and other supplies, with livestock, to the camps and forts along 
the old road from Ehrenberg to Wickenberg, and thence by other roads 
to points north, east, and south, and along these lines the introduction 
of the plant in question has been observed, as it has gradually spread 
through a large part of southern and southwestern Arizona. It appeared 
in the early seventies in the neighborhood of Oracle, and may have been 
brought from this point to Tucson, though this is not definitely known. 
At all events, it is now well established in various places about Tucson, 
including the Desert Laboratory domain. 

In February, 1906, a careful examination of Tumamoc Hill was made 
for the purpose of locating accurately the points where it had become 
established. As shown by plate 20, it was found to occur sporadically 
from the northeast corner of the domain to points east and south of the 
Laboratory and as far west as the wash. Most of the patches included 
a relatively small number of individuals, in some cases a single one, but 
there were a few areas of considerable extent, notably 5 and 6, where it 
had apparently been established for a period of years, and in places thickly 
covered the ground. Further details of distribution, as observed at the 
time indicated, are deposited in the Laboratory records for comparison 
with what may be observed of the progress of the invasion in future. 
Certain areas, each containing 100 square meters, have been staked out 
for careful observation from year to year. A chart (fig. 1) and a photo- 
graph (plate 21) of one of these indicate in part the method employed. 
The field-notes, which are a part of the permanent record, include the 
list of perennials, their number, size, and position, the distribution of 
the alfilaria on the square, its relation to other annuals, their relative 
abundance and probable competition, and still other matters which will 
facilitate comparison in subsequent years and aid in determining the 
advance or retreat of the alfilaria. 

Another species, Hordeum murinum, is of much interest, since it has been 
possible to observe what are apparently the very beginnings of its inva- 
sion, its habits elsewhere justifying the expectation that large areas of 
Tumamoc Hill will soon be occupied by this pernicious weed, which has 
become established in Tucson and the country adjacent, baffling every 
attempt to eradicate it. 

In April, 1906, an attempt was made to find as nearly as possible all 
the patches of this plant then in existence on the Laboratory domain. 


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SPALDING PLATE 21 





Permanently marked area No. 7, showing invasion of Erodium cicutarium. In center of area 
a large individual of Parkinsonia microphylla. 





Anchoring roots of a sahuaro, measuring 6.8 meters high and 39 cm. in diameter at the base. 


CAMPBELL ART CO., ELIZABETH, N. J. 


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LOCAL DISTRIBUTION OF SPECIES. 5a 


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Fre. 3; 
Area including 100 square meters, showing position of perennials (marked by a cross with 
initials of name) and alfilaria (marked by dots), February 21, 1906. 
Ac, Acacia constricta; Bh, Bigelowia hartwegwi; Ce, Calliandra eriophylla; Ds, Delphiniwim sca po- 
sum; Fd, Franseria deltoidea; Lb, Lycium berlandieri; Ov, Opuntia versicolor; Pm, Parkin- 
sonia microphylla; P, Philibertia sp.; Sp, Spheraicea pedata, 


As indicated by plate 22, 13 small patches were located, all of them at no 
great distance from the Laboratory building. Very probably a few other 
spots were occupied at that time by this grass, but if so, the essential 
fact remains that the points mapped include all that were readily found 
and represent, with substantial truthfulness, the very early stage of 
invasion then observed. As in the case of Erodiwm cicutarium, details 
of distribution are included in the Laboratory records for the purpose 
of reference in further observation of the invasion of this species. 
Invasion of many other species on Tumamoc Hill and in the adjacent 
valleys has taken place and is still going on. On the flood-plain near 
the river are extensive areas so covered with Xanthiwm canadense and 
Malva parviflora as to nearly obliterate all trace of the original vegetation. 
Along paths in the vicinity of the Laboratory Monolepis nuttalliana has 


54 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


taken complete possession of limited areas, and in spots where water 
has been used close to the building, Bermuda grass has made its appear- 
ance within the past year. 

Numerous other weeds, chiefly annuals, have come to occupy, more 
or less conspicuously, such ground as they have found available, especially 
on the flood-plain. While, as indicated, annuals and foreign weeds are 
most prominent as invaders, there are various perennials, particularly 
certain species of grasses, which present every appearance of invasion, 
either successfully accomplished or in progress. Among these are Hilaria 
mutica, and H. cenchroides, which, on the north slope of Tumamoc Hill, 
occupy extensive patches in the midst of other vegetation which they 
are apparently displacing. In order to determine this fact definitely, a 
number of permanently marked areas have been established for the pur- 
pose of continuous observation for a period of years, and maps and photo- 
graphs of these are kept in the records of the Desert Laboratory. 

In general, it appears that on the flood-plain the invasion of various 
species has resulted in their almost complete occupation of extended 
areas, so that the vegetation of these areas is essentially different from 
what it was in earlier days; but no such radical change is observable 
on Tumamoc Hill and the slopes adjacent. The invaders of the plain 
have taken advantage of changes effected by the hand of man. They 
found a fertile soil not fully occupied, and took possession with a thorough- 
ness that seems likely to be successful against competition for an indefinite 
period. On the hill and slopes, on the contrary, no such preparation 
for their invasion has ever been made, except to a limited extent in road- 
making, and their advance has been relatively slow and inconspicuous. 

Thus far the discussion has chiefly involved introduced weeds, but 
certain indigenous species have a history in some respects quite similar. 
One of the most interesting of these cases is that of Bigelowia hartwegit. 
On the flood-plain and adjacent slopes, this plant has been observed within 
recent years, almost covering the ground where before it was relatively 
inconspicuous. ‘This, according to Thornber, seems to have been closely 
connected with a period of abundant and well-distributed rains during 
the year 1905 and several months of the preceding and following years. 
During this period rain fell in sufficient quantities to insure the growth 
of large numbers of seedlings, which became well established and at the 
time of special observation in April, 1906, were apparently well beyond 
the critical period. At that time the parent plants could be observed 
quite scattering and several times larger than the seedlings of the new 
crop, while between them the young plants almost covered the ground 
and have ever since maintained the foothold then acquired. 


The various invasions that have taken place on Tumamoc Hill and 
in the adjacent valleys have necessarily involved competition. It is 


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DESERT LABORATORY DOMAIN AND VICINITY, SHOWING INVASION OF ERODIUM CICUTARIUM 


THE NUMBERED PATCHES SHOW THE POSITION OF AREAS OCCUPIED BY ALFILERIA IN FEBRUARY 


1906 


! 


LOCAL DISTRIBUTION OF SPECIES. 55 


often stated that with desert plants the struggle for existence is with 
their physical environment rather than with each other, but it is certain 
that within the area under consideration severe competition prevails 
between many of the plants which constitute its vegetation. For the 
purpose of observing the results of this process upwards of 20 different 
areas, each including from 1 to 4 square meters, were staked off on the 
Laboratory domain in the spring of 1907, and the relations of the plants 
included in them carefully noted, but until both observation and experi- 
ment have been carried much farther it will be impossible to give a satis- 
factory account of competition as it takes place between the plants here 
represented. The following notes will serve to indicate certain obvious 
relations worthy of record: 

(1) On the flood-plain, in ground where there is abundance of water, 
there.is not the slightest doubt that certain weeds, notably Malva parvi- 
flora, have successfully competed with various other species. In places 
this plant covers the ground so completely with its rank growth that 
other plants fitted to the same habitat are entirely unable, in ground 
occupied by the Malva, to reach maturity. In this case the unsuccess- 
ful competitors are probably destroyed by the dense shade of the aggres- 
sive weed, though it is likely that they also suffer directly through lack 
of food materials that it has appropriated. 

(2) Annuals, which after the winter rains grow in great luxuriance 
on Tumamoc Hill, exhibit in a striking manner the effects of competition 
between individuals of the same species and between different species. 
The phenomena, stunted growth, failure to produce seeds, ete., the same 
here as elsewhere, are so familiar as to require no detailed description. 

(3) Everything in the position and mode of growth of Hilaria cench- 
rovdes and H. mutica on the hill goes to indicate that we have here a case 
in which a perennial advances relatively slowly into ground previously 
occupied from year to year by annuals and holds it against their future 
occupation for an indefinite time. From what is seen elsewhere there 
is no doubt that this will be the history also where the Bermuda grass 
has gained a foothold in a few places near the Laboratory. 

(4) Study of the root-system shows a complicated set of relations in 
which a given plant may be in direct competition with certain species 
and not with others. Thus Cannon (p. 64) finds that Cereus giganteus, 
growing with Larrea and Parkinsomaa, is, to a great extent, relieved of 
competition on their part by the deeper penetration of their roots, while 
it comes into direct competition with various annuals, the roots of which 
occupy the same or nearly the same horizon. 

(5) But while it is evident that competition has had much to do with 
determining the preponderance of species in the various associations, it 
is also clear that in general it here stands in an altogether subordinate 
relation to adaptation. As a striking illustration, the zone of creosote- 


56 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


bush on the slopes above the flood-plain can not be thought of as gaining 
and holding its place through competition, in any just sense of the term. 
It has become singularly fitted to the hard conditions with which it is 
here confronted, and which, in times of excessive drought, try even its 
extraordinary endurance, and the almost complete absence of other 
perennials is due to their inability to cope with the severe test which 
only the creosote-bush has been able to meet successfully. In this zone, 
distinctively desert conditions prevail, competition between species is 
reduced to a minimum, and the struggle with environmental factors 
determines the case. None the less, the limited number of individuals 
of Larrea occurring on any given area of this zone, and the small size 
it here attains, are presumptive proof of severe competition between 
individuals of the same species. It may yet be possible by determina- 
tion of the available water-content of the soil in times of drought to find 
an expression for the possible aggregate growth of the creosote-bush on 
a given area, but this has not yet been attempted. 


By the invasion and establishment of such plants as those already 
described a succession results, and consequently at a given period the 
associations in which invasion has occurred present a more or less dif- 
ferent composition from that in a preceding stage, often becoming strik- 
ingly dissimilar to what they were before the invasion took place. 

In many of these cases, especially such as those described for the flood- 
plain, we are dealing with successions that are partial or incomplete 
and more or less ephemeral. In contrast with these are certain cases 
of invasion which result in successions of more permanent character. 
As already indicated, coincidently with the process of base leveling, 
several well-marked associations of plants in the vicinity of Tumamoc 
Hill are slowly advancing beyond the areas which they previously occu- 
pied. As the flood-plain of the Santa Cruz River becomes wider, at any 
point, the mesquite association also widens. As by the process of erosion 
the lower part of the hill becomes replaced by the long slopes or by the 
alluvial fans which form one of the most striking topographic features 
of the region, the creosote-bush association advances and takes posses- 
sion of the ground with its characteristic vegetation; and as the washes 
occupied by mesquite, catclaw, and Cercidium torreyanum shift their 
places, or are extended toward the hills, the plants of this association 
advance step by step with them. In each case an essential change of 
topographical boundaries has been promptly followed by the advance of 
each of the associations named into an extension of its appropriate habi- 
tat; in other words, we have in these cases a definite and well-established 
succession. 


LOCAL DISTRIBUTION OF SPECIES. 57 


HABITS AND STRUCTURES RELATED TO DISTRIBUTION. 


It is not the plan of the present paper to discuss in detail adjustments 
and adaptations concerning which our knowledge, though delusively 
comprehensive as to certain classes of facts, is still for the most part 
without a solid basis of demonstration based on experiment. It is desir- 
able, however, to direct attention to certain habits and structures exhib- 
ited by some of the most characteristic plants of the Laboratory domain 
that have enabled them, with the aid of concurring agencies and in har- 
mony with environmental factors, to reach their present stations and to 
establish themselves there. 

A whole biological group of plants may be so conspicuously distin- 
guished by some one habit or characteristic relation to a single factor 
of its environment as to materially change at stated times the landscape 
of the region which it inhabits. This, as we have seen, is the case with 
the two biological groups into which the annual plants of the desert 
country of the southwest are divided. Reference to these has already 
been made in an earlier section, and it is only necessary here to emphasize 
the fact that the winter annuals on the one hand and those of summer 
on the other have a single fundamental difference, which is based on 
temperature relations. ‘Thus we have a well-marked case in which distri- 
bution in time is correlated with a single factor, namely, temperature. 
How far the previous geographical distribution of these groups of annuals 
is related to their present habits is still a matter of inquiry. 

While, however, such characteristics may be observed in biological 
groups taken as a whole, they are, in general, more satisfactorily studied, 
especially in an experimental way, in single species. A few of the plants of 
the Laboratory domain that have been subjected to more or less extended 
observation and experiment have given definite results as to the extent and 
promptness of response which they habitually make to the action of a 
single physical factor. 

One of these is the ocotillo (Fouquerta splendens), which during times 
of drought is defoliated, but puts out leaves with astonishing rapidity 
in rainy seasons or when it is artificially supplied with water. The his- 
tory of an individual of this species for a period of several months during 
which it was under observation has been given by Cannon (1905). Exper- 
iments by Lloyd (1906) the following summer show that leaf-formation 
in the ocotillo may be artificially induced by supplying the plant with 
water through the aerial parts, though incidentally the fact is brought 
out that the response is more prompt when water is obtained by natural 
processes through the root. From the work of these two observers it 
becomes evident that the plant in question responds quickly to the influ- 
ence of a single factor, water, and that the amount necessary to elicit this 
response is capable of close quantitative estimation, and with greater 
perfection of methods might be subjected to exact measurement. 


58 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


Another plant, the sahuaro or giant cactus (Cereus giganteus), has given 
quite as striking results, and the extent of its response has been carefully 
measured. By means of an extended series of exact measurements, 
continued at intervals through a period of nearly four years, Mrs. E. S. 
Spalding (1905) has shown that the sahuaro, by a bellows-like action 
of its ribs and furrows, possesses a means of very perfect adjustment to 
varying amounts of stored water, and that its action is promptly set up 
when even a slight rainfall, after a period of drought, provides the neces- 
sary increase of water-supply. Plate 23 shows expansion and contraction 
between two points on opposite sides of a furrow of the giant cactus from 
March 3, 1906, to May 26, 1907. The broken portion covers a period 
in which no observations were made. Each horizontal space represents 
one day, and the days on which measurements were made are indicated 
by the dates. Each vertical space represents ¢; inch, and the figures 
at the sides the distance between the points in question in inches and 
sixty-fourths. Thus, on March 3, 1906, the points were 2}; inches apart, 
and on March 8, 24? inches, and so on. Below is shown the rainfall for 
the same period, each vertical space representing one-tenth of an inch 
of precipitation. When the rainfall amounted to only a trace it was not 
recorded. The complete record of precipitation for the period corre- 
sponding to the time for which the curve was drawn is given in table 
6, page 95. 

The giant cactus, rising as it often does to a height of 50 feet more or 
less, in the form of a gigantic fluted column, which may be simple or 
branched, is, mechanically speaking, a huge reservoir of water, subjected 
to the stress of high winds, and so constructed that for a long period of 
years it not only maintains securely its erect position and steadily con- 
tinues its growth, but also promptly expands whenever the soil is wet 
by rain, even for a short distance. The construction of such a tank 
represents an engineering feat which probably has no parallel in any 
artificial structure in existence, and the case appears still more remark- 
able when it is considered that this whole system of storage in an adjustable 
tank is dependent for its highest efficiency on the peculiarities of its root- 
system. ‘These are discussed by Dr. W. A. Cannon in the following pages. 





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LOCAL DISTRIBUTION OF SPECIES. a9 


THE ROOT-SYSTEM OF CEREUS GIGANTEUS.! 


The character and extent of the root-systems of desert plants, as well 
as the réle which they play in the distribution of these plants, are in the 
main not known. What their relations may be to the physical environ- 
ment, to the kind of soil, to the air in the soil and its temperature, and 
what to the varying amounts of available water of the soil, as well as to 
the root-systems of adjacent plants, and the influence of these and other 
factors on the presence of plants in their peculiar habitats are among the 
most pressing problems of desert botany that await studious inquiry. 
Something of these relations and something of their significance so far 
as field observations can point them, in one representative of the most 
perfectly adapted of all desert plants, will be summarized in the following 
paragraphs. 

Cereus giganteus has a fairly highly specialized root-system, which 
throughout the life of the individual plant has a close relation to its leading 
needs, namely, of adequate support, of sufficient water, and of proper 
aeration. The portions of the roots which respond to these needs are 
not fixed, but suffer gradual change, so that what is characteristic of the 
youth of the plant no longer holds for the mature form. In the young 
plant, the main root-system is that associated with the tap-root, which 
functions both for anchorage and for absorbing water. But with the 
growth of the plant, laterals arise which come to extend far from the 
parent root and which take over the function of absorbing water and 
leave that of supporting the stem to-the tap-root. This is the condition 
in plants about 20 cm. long and may be found in plants as high as 1.2 m. 
But in the larger forms the exposure to the impact of winds, which at 
times are of great violence, often, or perhaps always, makes such anchor- 
age as that afforded by the tap-root insufficient, so that later the final 
adjustment of the roots to external agencies is effected. This is the 
enlargement of at least the bases of the lateral roots, so that efficient 
braces are provided by which strains and stresses set up in the high sub- 
aerial portion are rendered harmless. We therefore find in a cactus 
6.8 m. high, for example, the following condition of the roots, which 
is the final one: In place of a single branched tap-root there are numerous 
rather slender and straight roots which form a brush or tuft (plate 21). 
In the case in question these penetrated the ground 77 cm., or a distance 
which was less than one-ninth the height of the main stem of the plant, 
or, including the branches in the estimate, one-fourteenth the entire 
length of that portion which was exposed to the wind. ‘The large laterals 
are the bases of the superficial or absorbing system of the plant, here 
functioning as a means of support as well. 


ee 


1Prepared by request and contributed by Dr. W. A. Cannon, Member of Staff of 
Desert Laboratory. 





60 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


As the tap-root, or the roots clustered about it and of the same char- 
acter, was no longer adequate for support, the lateral roots increased 
very greatly in diameter and assumed a part of the burden. Should 
the laterals die and decay before the destruction of the subaerial portion 
of the plant, as very frequently happens, this support is removed and 
the plant is easily thrown to the ground. From the circumstances given 
above it is clearly incorrect to make a sharp distinction at any period 
in the life of the cactus between water-absorbing and anchoring roots, 
since at no time does such distinction exist. So far as safe anchorage 
for the plant is to be counted a factor in limiting the distribution of Cereus 
guganteus it is apparent, therefore, that such can be attained wherever 
the permeable substratum is always firm and as deep as I meter or even 
less. This may be the case in certain places on the mesa-like slopes, or 
preferably on rocky places where crevices in the rock or spaces between 
bowlders afford depth of soil and additional support. 

The root-system of Cereus giganteus, in its relation to the substratum 
and to the roots of neighboring plants, may be best presented by giving 
a concrete example. The habitat of the specimen to be described is the 
eastern foot of the Tucson slope, running eastward from the base of the 
Tucson mountains; it is 0.5 mile west of the Laboratory. The ground is 
of a dark color and rather thickly strewn with dark volcanic bowlders 
of various sizes. A section of the ground where the cactus was growing 
shows the following characters: The uppermost soil, 30 cm. in thickness, 
is of malpais, which is derived from volcanic rock (basalt) ; in this stratum 
are bowlders of various sizes. Beneath the malpais stratum is a thinner 
one, 20 cm. in thickness, of caliche, in which also bowlders are embedded. 
Beneath the caliche is the solid rock. 

At this place the lay of the surface and its relation to the surrounding 
physiographic formations are such that no water comes to it from seep- 
age or drainage; the plants have to depend for their water-supply entirely 
on what rain falls on the particular spot. In the immediate vicinity 
of the cactus are to be found Parkinsonia microphylla, Larrea tridentata, 
Kramerva canescens, and near by Encelia farinosa and several species of 
cacti, notably Cereus fendleri and Mamillaria sp. The specimen of Cereus 
giganteus studied was a perfectly healthy and very vigorous plant 1.2 m. 
high and 35 cm. in diameter at the largest part. 

As already stated, the root-system of this species is of a double nature; 
it is in part deeply placed, which insures safe anchorage for the plant, and 
it is in part superficial, which provides a large absorbing surface and at 
the same time includes a relatively large area from which to extract the 
needful water-supply. The extent of the latter kind of roots is far in 
excess of that of the former. In the individual studied the deeply placed 
system consists of a single tap-root which goes straight down to a depth 
of 30 cm. when it gives off one branch that runs at right angles for a 


LOCAL DISTRIBUTION OF SPECIES. 61 


distance of 25 cm. ‘The main root, therefore, runs through the malpais 
to the caliche beneath it, where it abruptly stops (fig. 2). 

The main root gives off 6 laterals near the surface of the ground, which 
radiate from the stem in such a fashion that the ground included in their 
reach is fairly equally divided between them (plate 24). They extend 
from 1.5 to 5 m. from the stem; they are tough and rope-like and they 
branch but little. Four of these roots were not observed to give off 
branches for 1.5 m. of their course, and one root that branched freely 
at the tip gave off only 4 laterals in the course of 3 m. of its length. The 
superficial roots end in a tuft of delicate rootlets, nearly all of which die 
with the advent of the drier seasons. This habit of forming roots of 
limited growth and of shedding them when useless has been observed in 
other cacti also, notably in Opuntia versicolor, and in other 
plants of the desert. These are not confined to the tips of 
the roots, but may appear apparently at any place through- 
out its course. It is an interesting adjustment to desert 
conditions, inasmuch as it permits the plant to rework 
the ground which is not continuously moist but only inter- 
mittently so. By this means the area covered by the roots 


4 
3 i) Nhyt 1 Nh pe ty 
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IRVEE oe 
Vertical extension, semi-diagrammatic, of the root-system of an individual of Cerews giganteus 
which was 1.2 meters in height (solid lines), and that of a neighboring Larrea tridentata 
(broken lines). In the soil section the adobe is represented by the vertical broken lines; 


the hard pan, caliche, underlying it by the stippling, and the bed-rock by the slanting 
broken lines. One-twentieth natural size. 


is greatly restricted, but at the same time it is thoroughly exploited for 
its water. 

The superficial root-system penetrated the ground to a depth which 
was fairly uniform. For example, one root left the main root at the 
surface of the ground, and both it and its three main branches, with little 
deviation, ran 7 cm. from the surface. Another root ran from 3 to 10 
cm. from the surface of the ground, but gave off a branch which went 
straight down to a depth of 17 cm. Others of the superficial roots varied 
from 5 to 15 cm. in depth, and one dipped under a large bowlder, whose 
lower surface was 30 cm. deep, after which it ascended to about the depth 
it ran between the bowlder and the parent root. 


62 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


Where the larger of the superficial roots leave the main root they are 
relatively heavy, but taper rapidly until a small size is reached, which 
is maintained fairly well for long distances. A few measurements will 
indicate the character of these roots. One of the larger ones was 1.6 cm. 
in diameter at the base; another was 7 mm. in diameter 30 cm. from 
its base; another root measured 1 cm. in diameter 45 cm. from the main 
root, and it extended 2.5 m. farther from the place where the measure- 
ment was made. Roots not above 50 cm. in length were about 3 mm. in 
diameter at the base, but those longer, as, for example, 5:5 m., are some- 
what heavier, but still not very large. This one was 2 cm. in diameter 
at 30 cm. from the main root. 

In the area included within reach of the superficial roots were two 
perennials, namely, Parkinsoma microphylla and Larrea tridentata, and 
one of the longer cactus roots reached to the base of another Parkinsonia, 
not included here. During the rainy seasons the area is well covered 
with annuals, but as there were none present when the cactus was studied, 
their number and kinds are not known. 

The specimen of Parkinsonia was a small plant, but probably a rather 
old one, about 50 cm. high and 1.3 m. distant from the cactus. It gave 
indication of a severe struggle for existence, presumably owing to an 
insufficient water-supply. The following points were noted in the char- 
acter and distribution of its roots. 

A tap-root measuring 3.5 cm. in diameter at the level of the ground 
went straight down through the malpais and terminated abruptly on 
reaching the caliche. Several slender roots left the main one at distances 
varying from 5 to 10 cm. from the surface and ran in a fairly horizontal 
direction for approximately 10 cm. At a depth of 8 cm. a lateral root 
8 mm. in diameter was put out, which ran 20 cm. nearly horizontally, 
and then dipped rapidly, so that at a point 50 cm. from its base it was 
45 cm. beneath the surface of the ground. Thus it ran through the caliche 
and was lost in crevices of the underlying rock. At the tip of the tap-root 
there were several roots about 10 cm. in length. 

As opposed to the Parkinsonia, the specimen of Larrea was very vigor- 
ous and apparently perfectly normal. ‘The roots of this plant were traced 
as far as practicable, but owing to the age of the plant (it was evidently 
much older than the cactus) and to the consequent large extent of its 
roots, and further, as will appear directly, to the character of the root- 
system, it was not practicable to make so exact a study as was done 
with Cereus; however, the relations of the root-systems were followed out 
as carefully and as completely as possible. | 

The Larrea was about 50 cm. high and was much branched. It was 
50 cm. away from the cactus and 1.3 m. from the Parkinsonia. In the 
Larrea a relatively large mass composed of about three branched roots 
penetrated through the malpais to the caliche and there stopped (fig. 2). 


LOCAL DISTRIBUTION OF SPECIES. 63 


From this central root-complex about 5 laterals arose at a depth averaging 
15 cm. from the surface; these were traced from 1.5 to 3 m. without 
arriving at the tips. The course of all of these roots was a gradual descent. 
They passed on a gentle slope through the malpais and entered and passed 
through the caliche on the same slope, after which they reached the under- 
lying rock and ran along its surface. Where left, the roots were 60 cm. 
more or less beneath the surface of the ground. Besides these roots the 
main root-mass gave off numerous roots from 10 to 25 cm. in length. 

In general character, the roots of Larrea were quite unlike those of 
the Cereus. They were of a dark color, brittle, and rather large. One 
root, which was traced for a distance of 3 m., was at a point 60 cm. from 
its base 1.5 cm. in diameter; another 90 cm. from its place of origin and 
45 cm. beneath the surface was 5 mm. in diameter; and another root 
at a point 2.8 m. away from its base was 1.2 cm. in cross-section. At the 
“base all of the longer roots were about 2.6 cm. in diameter. 


We may now summarize the leading characteristics of the root-systems 
and the roots of Cereus giganteus, Larrea, and Parkinsonia and note more 
specifically their mutual relations. 

Cereus.—The main root penetrates through the malpais to the caliche, 
where it stops. In the young plant it serves both as an anchor for the 
plant and as an organ for the absorption of water. As the plant becomes 
larger, laterals which in a plant 1.2 m. high may extend as far as 5 m. away 
from the main root are pushed out near the surface of the ground. These 
roots are confined to the uppermost layer of soil, the malpais, and for the 
most part run within 10 cm. of the surface; they constitute the leading 
absorbing system. ‘They are of a light color and are tough and rope-like. 
In yet larger plants the bases of these roots are enlarged, and they func- 
tion in a mechanical way as important supports of the plant. 

Parkinsonia.—The main root goes through the malpais and stops when 
it reaches the caliche. The laterals leave the main root at a level 
somewhat deeper than in Cereus, and after running a short distance 
on a rapid decline pass into and run through the caliche and finally reach 
and run along the surface of the underlying bed-rock. Only the shorter 
roots, which branch out from the main root, remain near the surface of 
the ground. 

Larrea.—The main root complex enters the malpais and goes straight 
down to the caliche, where, asin Parkinsonia and Cereus, it abruptly stops. 
The laterals run for a short distance in the malpais, at no place nearer 
the surface than 15 cm., and enter the caliche, which they pass through 
on a slope, and reach and run along the surface of the rock beneath. As 
a whole, the roots of Larrea were larger than those of Cereus, but this 
may have been due wholly to the difference in age of the two forms. In 
both Parkinsonia and Larrea the main root-system was the leading sup- 


64 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


port, which was also true for the relatively young Cereus, and it is prob- 
able that this system in the two former plants always subserves this end. 

The relations of the root-systems of the three perennials are shown 
in the vertical and the horizontal charts. The absorbing roots of Larrea 
and of Parkinsonia were not completely exposed, although they were 
dissected sufficiently to show that the roots of neither occupied the same 
soil horizon as those of the cactus. Indeed, physical contact between 
the three plants was demonstrated in one instance only. In this case 
one of the longer roots of the cactus sent out several smaller roots which 
completely encircled the root crown of Larrea (plate 24). The superfi- 
cial roots of Cereus did not extend in the direction of Parkinsonia, but 
the tips of one of the longest of these were traced to near the base of a 
neighboring Parkinsonia which was on the circumference of the area 
investigated. The course of the roots of this plant was not followed. 

Although it thus appears that the roots of Cereus are so far removed 
from those of the other two plants that the most intense competition 
for water, such as would occur if the roots lay in the same level, probably 
does not take place, this does not mean that the plant is free of compe- 
tition. The annuals, which are to be found in this area at different seasons 
of the year, do not, so far as observed, send their roots more deeply than 
the malpais, and, therefore, although their roots include a greater verti- 
cal range than those of the cactus, they compete actively with its roots 
in the uppermost portion, at least, of the soil. 

The superficial nature of the absorbing root-system of the Cereus brings 
the plant into relations with the temperature of the ground and the rain- 
fall which most of the other perennials do not enjoy. ‘The influence 
of the temperature of the ground is not so obvious as that of the rainfall, 
but nevertheless it may be found to be of considerable moment as a factor 
controlling or limiting the distribution of the Cereus, in the absorption 
of water, and probably in other ways. Reference to the records of the 
temperature of the ground at a depth of 15 cm., within which the entire 
absorbing system of the cactus is placed, which have been made at the . 
Desert Laboratory since 1905, indicate that frequently the summer tem- 
perature of the soil at that depth may reach and exceed 37° C. and that 
the winter temperature may fall to 10° C. or below. ‘Temperatures 
nearer the surface, where the main absorption root-system is placed, are 
not given, but must be higher in summer and lower in winter than the 
extremes given for the greater depth. 

By these conditions of soil temperature, which increase the xerophilous 
conditions, Cereus is subject to an environment quite distinct from that 
affecting either Parkinsonia or Larrea, and which is directly traceable 
to the superficial nature of its root-system. This circumstance, when 
related to the penetration of the rains, is seen to be of even greater impor- 
tance. All heavy falls of rain, as those 1 inch or over, penetrate the soil 


PLATE 24. 


\ 
- 
“ ) 
\ ote { 
' e 4 
1 , 
J i oe 
4 } =a a 
v4 fs 
\t ' 
Vu, ‘ 
eed ‘ ! 
~~ & ee “ 
—e~ 2 ee ‘ San f ts “5 
‘ Teese AS==: \ -- 
Zak aes 
Pa d x AN 
\ ay 
r 
mS, vat = 
—- \ 
~ \ yp, 
s 
t 
b ry 
‘ ‘\ 
\ ss 
\\ No 
\ <a 
\ ’ BSS 
, 4 
\ 4, 
\ { \ 
? 
re R 
. os 
4 ‘ 
“ ‘ 
- 
/ a 
vi Sy 
. 
, =e orn 
pe \ as ra © St ae 
[= ? og eta ’ a 
! a Se ae 2 
‘ ‘ t 
‘ ) <e 
+ ‘. “ . 
ii ' x 
‘ “= ie \ 
bas) 
ee \ 
’ 7 A 
eit / Nepal, 
\ , ey re 
. 1 ~ 
, ‘ 
‘ ' 
e ' 
. 
eS 
Cae 
. ’ 
\ ’ 
. . 
1 ' 
e ' 
‘ \ 
\ 


Chart of the root-system of sahuaro (solid lines), creosote-bush (dotted lines), and 
Parkinsonia (broken lines). 


One thirty-second natural size. 


a 


gre iw wae on 





LOCAL DISTRIBUTION OF SPECIES. 65 


at a sufficient depth to be available to all plants, whatever may be the 
type of root-system, but rains of much smaller amounts probably benefit 
to a limited degree, if at all, the perennials with deeply lying roots. 

That different species of cactus are able to absorb water from a pre- 
cipitation of 0.5 inch (12.7 mm.) has been frequently observed. For 
example, on the night of October 17, 1907, 0.54 inch (13.7 mm.) of rain 
was recorded at the Desert Laboratory. ‘Twenty hours afterwards the 
flat stems of Opuntia discata, which previous to the rain were wrinkled 
and shrunken, were observed to be plump and smooth. An instance 
is given by Mrs. Spalding (1905) in her study on the mechanical adjust- 
ment of the sahuaro to varying quantities of stored water, in which after 
a rain of o.5 inch, the stems expanded steadily for three weeks. It has 
been observed that with a rainfall of 0.54 inch (13.7 mm.) moisture from 
it may be detected at a depth of 4 inches (10.16 cm.), and from what 
has been said above regarding the nature of the absorbing root-system 
of Cereus giganteus water at this depth, as well as a much less depth, would 
be available to the cactus. 

A review of the rainfall by days at the Desert Laboratory from May, 
1904, to December, 1907, shows that there have been many such small 
rains. There are short periods of precipitation followed by longer ones 
of dryness in spring and autumn which may not be of sufficient amount 
to support the annual vegetation, and which would directly benefit only 
the plants with a root-system similar to that of Cereus giganteus, so shal- 
low as to be within reach of such slightly penetrating rains. 

Of whatever causes may act to bring about the superficial placing of the 
root-system of Cereus giganteus and other cacti, if a direct response to 
external physical agencies, the two following suggest themselves as being 
worth considering. These are: First, the average depth of the penetra- 
tion of the rains; second, need of proper aeration, although the temper- 
ature relations may also be important here. 

As mentioned in the foregoing, a study of the rainfall at the Desert 
Laboratory indicates that many of the rains do not penetrate to a greater 
depth than that of the absorbing roots of the cacti. But that this is not 
the determining cause is probable from the fact that the heaviest rains 
of the year, those of summer, occur at the season of most active plant 
growth and penetrate the ground to a depth much beyond the cactus 
roots. This conclusion is strengthened by the fact that in regions south 
of Tucson, where fleshy cacti are most abundant, the rains are seasonal 
mainly. How far, on the other hand, the need of proper aeration controls 
the placing of the absorbing root-system is undetermined, but it is not 
unlikely an important factor. 

The behavior of the roots of plants growing in porous pots and in pots 
impervious to the air, as well as the plugging of drains by roots, are familiar 
examples of the direct influence of the need of aeration on growth of roots. 


66 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


It is possible that an analogous condition obtains in the fleshy cacti par- 
ticularly, in which the formation of leaves, those most active agents for 
the promotion of gaseous exchange, is entirely wanting. This circum- 
stance, together with the large size of the plants and their intensely 
xerophytic habit, would cooperate to reduce gaseous exchange with the 
surrounding air to a minimum and might make necessary the most 
favorable conditions of aeration for the roots, which would be found not 
far beneath the surface of the ground. 

The role played by the roots in the distribution of Cereus giganteus 
is probably a most important one, but its precise value remains to be 
determined. It naturally must be associated with the habit of the plant 
as being adapted to mild climates as well as dry ones. This must be taken 
into account, whether the general distribution or the local distribution 
merely is being considered. ‘The fact that the upper stratum of soil, where 
the roots lie, is subjected to great changes of temperatures and would 
afford little protection to the embedded roots and none to the subaerial 
portion of the plant would be an important factor and possibly a defini- 
tive one at the extremes of the range of the cactus, and may be of much 
weight in restricting the local distribution of the plant as well. For 
example, as elsewhere pointed out, in the vicinity of the Desert Labora- 
tory Cereus giganteus is most abundant on southern exposures, and in 
the neighboring Santa Catalina Mountains, where it is to be found at an 
altitude approximating 4,500 feet, it is wholly confined to this exposure. 
Temperature records made of the northern and the southern exposures 
indicate the importance of this thesis. 

Another factor limiting the range of the cactus is to be found in the 
entire dependence of the plant for its water-supply on the surface-water. 
The rains, therefore, must be of sufficient amount and kind to penetrate 
to the absorbing roots; and where this fails the plant will not be found. 
Although not demonstrated, the opposite extreme is probably as great a 
barrier, possibly from its effects of depriving the plant of proper aeration, 
as was mentioned above. And in this cause may possibly be found one 
of the factors operating to prevent the presence of Cereus on the heavy 
adobe soil of river-bottoms, where one of the imperative needs of the plant, 
that of safe anchorage, were the root-system a deep one, could be met. 

From the relations which the superficial character of the roots of Cereus 
hold to the temperature of the air, to the rainfall, to the plant’s need for. 
proper aeration and support, we find, therefore, important factors which 
must be considered in studies on the adaptation of the plant to certain 
types of habitats and in studies on its general distribution as well. 


CHAPTER Ill. 
ENVIRONMENTAL AND HISTORICAL FACTORS. 


THE GEOLOGY OF THE VICINITY OF THE TUMAMOC HILLS.! 


This contribution presents in an informal way the results of the study 
of the Tumamoc Hills, a group of three low basaltic hills about a mile 
west of the business center of the city of Tucson, Pima County, Arizona. 
Tumamoc Hill and the adjacent slopes to the west are the site of the do- 
main of the Desert Laboratory of the Carnegie Institution of Washington. 

I have been requested to preface the geological description of this 
particular locality by a brief discussion of some of those modifications 
of geological processes and deposits induced by moderate aridity that are 
prominently developed in the southern part of Arizona. Attention will be 
directed to subaerial or land deposits; a complex group which it is neces- 
sary to separate from aqueous deposits, both lacustrine and oceanic, 
if sound geographic and climatic deductions are to be drawn. ‘The wash 
conglomerates, volcanic tuffs and lava flows described later represent three 
important varieties of land deposits, viz, torrential wash deposits, eolian 
deposits, and volcanic flows. 

On account of the intimate relation of the geology of the land to botany, 
and because I am convinced that climate is as important a factor in 
determining the detrital covering as the vegetal covering of the earth, 
and because it is within reasonable expectation that the twentieth cen- 
tury will see the determination of the climatic history of the past as a 
result of the critical study of land deposition, I assume that botanists 
will be interested in a short summary of those recent geological studies 
which, using graphic language, have resulted in the discovery of the 
geology of the land. 

In this introduction, space permits only an outline of the results of these 
lines of research and forbids any close examination of the data or meth- 
ods used. Instead of a complete bibliography of this subject, I shall 
note only those important contributions that will give some idea of the 
method, scope, and results of these investigations. I shall omit the 
consideration of the development of the scientific study of topography 
and of glacial deposits, which might well be taken up in this connection, 
the results of which are among the most important obtained by geology 
during the latter part of the nineteenth century, and which drew the 
attention of geologists from the ocean to the land. As Europe is behind 





1This section, pages 67-82, was prepared by request and contributed by C. F. Tolman, 


B. S., Professor of Geology in the University of Arizona. a 


68 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


America in the study of these newer phases of geology, attention will 
be directed to the development of these ideas in the latter country. 

In 1897 Davis' presented an excellent summary of some of the cri- 
teria to be applied to distinguish lacustrine from fluviatile sediments, 
although of course the ideas developed were not wholly original,? and 
Gilbert, Hatcher, Johnson, Grabau, Barrell, and others? have, by an exten- 
sion of these and other principles, rescued a number of the clastic deposits 
of the eastern United States from the domain of the ocean, and have so 
changed the ideas in regard to the Tertiary and the Quaternary* that these 
two periods may be defined as an age of subaerial deposits instead of an 
age of lakes. It is thought that throughout this time the conditions in 
western North America were much as now. ‘There were periods of local 
humidity when deposition in lakes was important, times of humidity and 
cold when lakes and glaciers were formed, and periods of warmth and _ 
aridity when the subaerial deposits, always important, took complete 
ascendancy over all other forms of deposition. Volcanic activity was 
pronounced, and marine sedimentation is recorded largely at the margins 
of the continent. 

This new concept of the importance of sedimentary deposition upon 
land surfaces made possible the next step, viz, the recognition of climate 
as one of the factors controlling subaerial deposition, and, as a corollary, 
the reading of the climates of the past times by the evidence presented 
in their respective deposits.> It is scarcely necessary to refer to the 
Asiatic expeditions of the Carnegie Institution of Washington as the best 
example of a critical study of glacial, fluviatile, and lacustrine deposits 
which has developed climatic deductions of great importance. 

A moment’s consideration will show that a relation exists between 
climate and erosion and deposition, but that this relation is not simple. 
For example, deposits piled at the foot of a mountain of considerable 
height will resemble in some particulars those formed at the foot of hills 
under the intense atmospheric and torrential action of a more arid cli- 
mate. Still further, climate itself is a complex, not a simple factor. 
Assume, for instance, a certain moderate number of inches of rainfall 
and then increase the temperature, and one discovers that the gentle 
erosion retarded by a covering of vegetation gives place to the intensive 
erosional and depositional action of the semi-arid climate. Again, by 





*Davis, W. M.: Is the Denver Formation lacustrine or fluviatile? Science, n. S., 
VI, 619. 

“Walther, J.: Einleitung in die Geologie, 1904. 

*Grabau, A. W.: Types of sedimentary overlap, Bull. Geol. Soc. Am., xv: 567-636, 
1906. Barrell, Joseph: Origin and significance of the Mauch Chunk shale, Bull. Geol. 
soc! Ani, XVIN, 449-476. Barrell, Joseph: Relative importance of continental, lit- 
toral and marine sedimentation, Jour. Geol., xrv, 316-356, 430-459, 524-568, 1906. 

*Chamberlin and Salisbury: Geology, 3, 193-194, 196-215, 243-248, 258-276, 296-318, 
472-483. 

Baers especially, Ellsworth Huntington: Some characteristics of the glacial period 
in non-glaciated regions, Buli. Geol. Soc. Am., XVIII, 351-388. 


ENVIRONMENTAL AND HISTORICAL FACTORS. 69 


lowering the temperature, the disruptive action of frost and the repres- 
sion of vegetation may under certain conditions develop intensive action. 

It does not seem probable, therefore, that theoretical analysis of the 
complex relations that obtain between climate and deposition will accoim- 
plish what it has in the case of deposition by running water or by glacial 
action. The problem will be solved by detailed studies in each region. 
In each case the disturbing factors must be evaluated and the intensity 
of each process gaged.! 







IDEAL SECTION THROUGH A PLAYA 
A= Angle of rock surface 


” 4 erosloTe 


B= 
C= 4 4 slope deposition % 





















Fic. 3. 


One of the most inviting fields for such a study is the “bolson”’ or 
undrained area developed under conditions of aridity. The typical 
bolson? is an expression of aggradational processes and is excellently 
portrayed in the smaller undrained flats of southwestern Arizona. An 
idealized section through a bolson is attempted in figure 3. The angle A 
represents the slope of the top of the mountain chain or peak. This 
upper surface is often parallel to the rock structure and is therefore flat 
or sloping as the strata are horizontal or inclined. The higher moun- 
tains reach up into a region of forests and abundant grasses which pro- 
tect the surface and preserve the flat top. The angle B is the slope 
developed by the attack of torrential precipitation upon the rock struc- 





1Since writing the above I have received the Journal of Geology, xvi, No. 2, which 
contains the first installment of an article under the title ‘‘Relation between climate 
and terrestrial deposits,’ which is to be continued in Nos. 3 and 4. It appears that 
Professor Barrell has not been deterred by the difficulties inherent in any general anal- 
ysis of the relations between climate and deposition. The portion published indicates 
clearly the great value of his contribution, and the completion of his article is awaited 
with impatience. 

2Hill, R. T.: Topog. Atlas of the U. S., Folio 3, U. Se Gra tooo.) Kmever: Cas. 
Bolson plains and the condition of their existence, Am. Geol., xxxIv, 160-164. Tight, 
W.G.: Bolson plains of the southwest, Am. Geol., XxxXv, 271-284. 


70 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


ture. It is therefore a function of both rock composition and structure 
and of climate. Strange to say, erosional attack is given strongest topo- 
graphical expression in regions of very scanty rainfall, where the torrent 
of every dozen years takes everything before it (plate 25). Slope C is 
the most striking and typical of the topographical features of the semi- 
arid regions, and not only the larger ranges surrounding the bolson, but 
also each little individual hill appears, as it were, mounted on a sloping 
pedestal (plate 25). These slopes have often been described.’ Herrick? 
has aptly named the extensive slopes of low gradient in New Mexico 
“clinoplains,’’ and Ogalvie® has called the talus apron of the individual 
hills “conoplains.”’ 

While these names have much to commend them, there seems to be a 
need for a simple term for this extensively developed topographic feature, 
a name that will go with such apt and commonly accepted terms as mesa, 
playa, bolson, etc. I therefore suggest the term slope for the inclined 
plain, and the word wash to cover the various forms of outwash deposited 
on the slope. 

It would not be necessary to emphasize the subaerial nature of the wash- 
deposits of the slopes, were it not for the fact that erroneous interpre- 
tation has recently been given them. They are without question the 
depositional phase of the erosional activities above. Running water 
plays a varying role, having no part in the formation of the symmetrical 
slopes under certain arid conditions, temperature change and gravity 
alone being involved. Except for the steepening of the angle close to the 
base of the mountains, these plains keep their even gradient sometimes 
for miles (plate 25). 

The undrained flat in the center is the playa. With increased pre- 
cipitation or decreased evaporation it becomes a lake. Its material is 
almost entirely wind-blown, deposition being most active during periods 
of water occupancy, when the dust from the mountains and slopes is 
caught by the water-sheet. 

In bolson topography there are all gradations, from those showing 
marked development of these features to those in which some of the 
features can scarcely be recognized, on account of milder climatic con- 
ditions, partial drainage by some master stream that has conquered 
the desert conditions, or the lack of the tectonic features necessary to 
initiate the cycle. 

It needs but a cursory investigation to suggest that the past climate 
can be read, provided that the deposits laid down in the center of the 
bolson under the varying conditions of no water (desert flat), playa, and 
expanding and contracting water-sheet can be separated from the sur- 





“Blake, W. P.: Some salient features in the geology of Arizona, etc., Am. Geol., 
XXVII, 167. 

* Herrick, C. L.: The clinoplains of the Rio Grande, Am. Geol., xxxrv, 376-381. 

*Ogalvie, L. H.: The high altitude conoplain, Am. Geol. xxxvI, 27-34. 


SPALDING PLATE 25 





Volcanic hills in great basin, near Hazen, Nevada. 





Typical slope west end of Santa Catalina Mountains from the north. 


CAMPBELL ART CO,, ELIZABETH, N. J. 


x pele 


ae vr a 


) 


e, « 
oe. t 
¢ 


THE LIBRARY 
OF ak ae ee 
WHIVERSITY GF ELLINGIS | 





ENVIRONMENTAL AND HISTORICAL FACTORS. it 


rounding wash-deposits of the slopes. The studies now in progress suggest 
that the critical line between erosion (above) and deposition (below) is 
a function of climate. It is a fact that in many cases the upper edges 
of the slopes are being dissected. Here, however, the changes are more 
complicated than in the former case. 

I shall content myself with the presentation without discussion, of a 
bare list of the criteria I have found to be available in this special case 
to distinguish the two kinds of deposits. I shall only consider particu- 
lars in the physical characteristics and shall omit differences in chemical 
composition and fossil content, which have been discussed in the litera- 
ture of the subject. . 

I have been asked for a statement concerning the origin of “caliche,”’ 
in the light of recent studies in the vicinity of Tucson. Little can be 
added in the way of description to the excellent account of Dr. Blake.! 
He applies the name to the surface crust, mainly of lime carbonate, that 
develops under the conditions of mild aridity in Arizona and northern 
Mexico. I think that this definition should be followed, and as suggested 
by Lee the term ‘‘cement’’ be applied’ to the crystalline matrix of the 
desert fill, a crystalline, calcareous cement-conglomerate, often containing 
considerable amounts of selenite and hyalite, and a strongly gelatinizing 
silicate, probably a zeolite. 

Dr. Blake assigns the source of the caliche to the ground-water drawn 
upward to the surface by capillary action and evaporated there, finding 
support for this conclusion in the fact that the well waters in the vicinity 
of Tucson are charged with the mineral constituents of the caliche. The 
conclusions of Dr. Blake have been verified by Ransome’ for the vicinity 
of Bisbee, and by Lee* for certain deposits in the Salt River Valley. 
While it is not impossible that this explanation may fit certain cases 
where the water-level stands within a few feet of the surface, it certainly 
can not be applied to the Tucson region to meet whose conditions the 
hypothesis was framed. Here, where the caliche is thickest, the water- 
level is from 60 to 120 feet below the surface, and it is a well-known fact 
that in fine soils capillary action can only develop a head from 6 to 12 
feet. Furthermore, in all observed cases where the ground-water stands 
close enough to the surface to furnish a supply for evaporation, the sol- 
uble- alkali salts and not caliche are deposited. 


1 Blake, W. P.: The caliche of southern Arizona, etc., Am. Inst. Mng. Engs., XxxI, 


—220. : ; 
She W. T.: Underground waters of the Salt River Valley. Water Sup. Papers, 


Pe, INO: 136. ; ; 
Se B. ie The Geology and Ore Deposits of the Bisbee Quadrangle, Arizona. 


Prof. Paper 21, U.S. G.5., 74. 
Sree a Welt slic, TT. 





72 


DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


TABLE 2.—Particulars in Physical Structure which may be used to Separate Wash, Playa, and 


Lake Deposits. 














The wash deposits. 


The playa deposits. 


The lake deposits. 





Stratification. 
(1) Stratification 
developed. 


very strongly 


(2) Rapid variations in coarse and 
fine material in both a vertical 
direction and in a direction of 
the strike. 


(3) Bowlders in matrix sand or 
clay. 


(4) Thin layers of flattened, pol- 
ished pebbles overlying dust or 
clay. (Indicates accentuated 
arid conditions.) 

(5) Plunging stratification, small 
pockets of perfectly sorted sand, 
trains of coarse material, varying 
in size up to bowlders 1o feet in 
diameter; all developed in the 
courses of the transient torren- 
tial streams. 

Sizé of material. 

Torrential drainage often has the 
capacity to carry bowlders to to 
20 feet in diameter. 

Distribution of coarse and fine. 

Coarse in the upper edge of the de- 
posits fringing the mountains 
and reaching far down in the 
stream channels, 

The upper limt. 

The upper margin does not lie ina 
horizontal line. It extends up 
the valleys. 


Shape of constituents. 

The bowlders are angular and semi- 

rounded. 
Slope. 

Increase with approach to source, 
farther out very regular. This 
long regular slope is not dupli- 
cated in any other kind of depo- 
sition, 

Other phenomena. 

Wind ripples often developed in 

sandy layers. 





(1) Deposits of mud and 
clay without marked 
stratification. 


(2) Occasional layers of 
single thickness of flat- 
tened pebbles, fitted 
together and polished by 


the wind indicates pro- | 


longed desiccation. 


Very fine sand and clays. 


a) Si teparie 9 © ‘che yore) Wmeiey we Sige fours. 2 ae 


Deposit limited by a 
horizontal line, which 
fluctuates with every 


change in the relation 
of precipitation toevap- 
oration. 


1S Fe. oe wien ew 6) ei he Wel a) a Keen, ove at Sure te Sole 


Practically horizontal. 


Mud cracks, fossil prints, 
etc. 





(1) Stratification marked 
at margin and showing 
less development away 
from shore. 

(2) Typical shore forms 
and stratification due 
to shore action. 


| (3) Deposits sorted to a 
_ uniform size. This is 
never accomplished in 
wash deposits except 
in local pockets in the 
larger stream courses. 


Material larger than cob- 
ble not common. 


At the shore-line. 


Bounded by a horizontal 
line along which may 
develop the erosional 
features of the shore- 
line. 


Bowlders are rounded or 
semi-rounded. 


Decided slope only at 
shore-line and then not 
equaling that of the 
wash. 


Sand dunes can only de- 
velop to any amount 
where there is perfectly 
sorted beach sand to 
furnish the material. 











ENVIRONMENTAL AND HISTORICAL FACTORS. 73 


Professor Forbes, of the Agricultural Experiment Station of the Uni- 
versity of Arizona, has kindly given me the following statement of his 
conception of the origin of the caliche: 

Caliche is a mixture of (probably) colloidal clay and carbonate (mainly) of lime. These 
two constituents, one in solution in the carbonated rain-water, and the other in suspension, 
are carried from the surface of the desert soils to the depth to which occasional rain pene- 
trates, which in this region varies from a fewinchesto3or4feet. At the general average 
level at which the wetted soil dries out through the desiccating action of an arid atmos- 
phere a more or less compact caliche stratum is formed. In situations where the soil 
surface is filled in from time to time new caliche strata are formed below each new soil 
level, the more recent formation therefore being above, the older below, in such sec- 
tions as are revealed by well-borings near Tucson. 

This explanation with some modifications will account for the occur- 
rences I have observed. Professor Forbes recognizes only the descending 
percolation of the water, but there is often a subsequent drawing to the 
surface by evaporation, whenever the drying out of a soil after a rainfall 
overtakes the downward percolation of the water, and taps the reservoir 
of moisture that is retained, even under arid conditions, just under the 
porous soil surface. To develop crusts there must be some ready supply 
of calcareous matter, and therefore no active underground drainage to 
remove the same. In the case of the Tumamoc Hills the source is the 
igneous rocks, especially the amygdaloidal cavities, as described later. 
In the vicinity of Tucson, the widely distributed Paleozoic limestones 
were the original source of much of the calcareous cement of the fill. 
As the gravel deposits are accumulated the caliche layers are built into 
them and slowly recrystallized by the percolation of the deeper waters. 
The development of the caliche is a rapid process, and in this it is in keeping 
with the acceleration of geologic action under aridity. In one place under 
observation the caliche was broken up and mixed with lime and brick 
left after building operations, and the mixture leveled and packed. No 
vegetation was allowed to grow and the ground was flooded frequently 
to keep the surface packed and lay the dust, etc. Within two years’ time 
there had developed 2 inches of typical caliche crust within half an inch 
of the surface. In other places, where the ground was not flooded, the 
new caliche is recognized with difficulty. 

The body of this article is divided into three divisions, viz, Topography, 
Geology, and Petrography. Prof. F. N. Guild’ published a microscopic 
description of the rocks of this area in 1905. He has kindly consented 
to examine some new material and revise his descriptions. His contri- 
bution appears under the head “ Petrography.” 


TOPOGRAPHY. 
The largest of the group of three hills already mentioned has been 
rechristened with its old Indian name, Tumamoc, by the staff of the 
Desert Laboratory. It was formerly known as Turtleback. Of the two 





*Guild, F. N.: Petrography of the Tucson Mountains, Am. Geol., Xx, 313-318. 


74 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


smaller hills the most easterly of the group is Sentinel Hill and the third, 
which is a ridge between the two, is unnamed. ‘Topographic analysis 
leads to the following divisions, which will be described in order. 

(1) The Tucson Mountain Slope.—This extensive slope reaches down 
from the Tucson Mountains several miles to the west. It comprises 
more than half of the height of the range above the Santa Cruz River, 
to which it descends. 

(2) The Tumamoc Hill Slope-—The Tucson Mountain slope passes 
under the superposed Tumamoc Hill slope. The two meet on a line 
marked approximately by the main gully southwest of Tumamoc Hill. 
The lower limit of this slope is well defined on three sides by sharp con- 
tact with the Tucson Mountain slope, and on the east by the Santa Cruz 
alluvial flat. The upper limit, however, practically defies definite limi- 
tation, culminating in irregular talus slopes reaching up to the top of the 
hill in places, while again the lava rock extends down to the Tucson 
Mountain slope or the alluvium. 

(3) The rock surjace——This can not always be separated from the 
débris-built slope, as there is more or less volcanic rubbish, even on the 
tops of the hills. In all the hills it has a moderate dip toward the north, 
but faces the south with a much steeper declivity, developing small cliffs. 
The northern rock surface is diversified in a minor way by small hum- 
mocks 5 or 1o feet high, formed by a turning-up of the edges of the 
cooled surface of the lava by a second movement. Talus patches lie 
below these cliffs. 

(4) The alluval plain.—This is the product of the Santa Cruz “ River,” 
a small intermittent stream (over 100 miles in length, however), which 
until a few years ago was in the depositional stage, but, aided by an 
abandoned ditch, succeeded in starting a new cut which it is now extend- 
ing southward. 


GEOLOGY. 


The Tumamoc Hills are the product of the final (probably Quaternary) 
stage of the extensive Tertiary volcanic activity of this portion of Ari- 
zona. The interesting features are presented within an area of about 2 
square miles in the immediate vicinity of the above-mentioned hills. 
Originally most of the surface was mantled with a covering of volcanic 
blocks, averaging roughly between 6 inches and 3 feet in diameter, and 
under those conditions the first impression would have been that the 
hills were formed by explosive action. The industry of the Mexican 
teamsters who have transported most of the surface covering to Tucson, 
where it is in demand for foundation purposes, has developed unusual 
facilities for observation, the results of which suggest that many of the 
déebris-choked lava’ hills have a more interesting and complicated struc- 
ture than is suggested by their exterior. With the accompanying topo- 
graphical and geological maps (plates 26 and 27) before the eye, it seems 


a ad aaa Ran Pale hs vor ee ie We pa Pe zy) -— i ‘, \ f 
7: . : ‘3 Qe 7 fe 1 > + jhe i ot hy a ih, ¢ f my nal ags a . 
































: ef STAs a rho ae? iy i ¥ ay es A ; 
ae 1 dia ae J ; 
a 7 +) ip" se 7 
i 
’ } 
aie piri iat AT r 
; 
; 
i 
MOR G 
| 
. * 
. o. 
ae | N? D 
. \ x ; ’ J | : 
| . maT ; “y ri " ~ +6 | naga eS 
halls 1 ie ee ae 
Ses , i tae “ rhe i st 
i "yj ii ; : ; Se +s a. i \ 
‘f is ee beim NG 
i ene ots | ouvcanry AF wars i nie? tm 
> =)! F ‘ ‘ ' ; peg Or et a | 
‘ete ine ae q ale ' . hal ee Gl a WS ae Rtn 9) Cana : d | 
a ; 4 a ) > A 
aa t he 
28S: 4 
: os 


; a qian, *% hi daca i PRR Hie 


PLATE: 26 















LEGEND 


o Soil Stations 


HOSPITAL O 


) 





| | 

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| | 

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7 apwhaes 

oo ae 
| 

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Ls 
ie ES PE — oh: ) - oa. =o. ina 








Surveyed October, 1906 


-~ Cc. B. C! RESERVATION AND VICINITY OF THE DESERT BOTANICAL LABORATORY OF TUCSON, ARIZONA A HORN 8 CO BALTIMORE 
Wins Bias } Topographers | 


SCALE: 335 INCHES PER MILE 


wan 
é 


ENVIRONMENTAL AND HISTORICAL FACTORS. 5 
unnecessary to describe in detail the distribution of the formation, or to 
divide the descriptions under the numerous subheads usually employed 
in the analysis of more extensive areas. I shall, therefore, combine in an 
informal way the development of the geological history of the region, with 
descriptions of the rocks, their outcrops, and their structural relations. 

The andesite flow.—The supposedly oldest rock of the Tucson volcanic 
series is an andesite, described microscopically as a “‘mica-andesite.”’ It 
is covered on the north, west, and south by rhyolite flows, is intruded by 
basaltic plugs, is covered by small separate basaltic flows in places along 
its eastern edge, is largely hidden under a covering of desert wash, and 
is bounded on the southeast by the Santa Cruz alluvium. The structure 
of the flow has not been deciphered. Its crystalline character suggests 
a very thick flow, or perhaps it is intrusive in part. It appears on three 
sides and underlies the basalts of the Tumamoc Hills. 

The rhyolite extrusions—The next event of the district was the outflow 
of many sheets of rhyolite. These are only recorded indirectly in the 
-region mapped by the pebbles in the later conglomerates. The back- 
bone of the Tucson Mountains consists of this rock, which includes not 
only typical rhyolite, and rhyolite breccia (the fragments of which are 
products of explosive action and foreign material picked up during the 
extrusive processes), but also all the grades between a vitreous rhyolite 
and a mud flow, the variations being formed by different mixtures of 
lava, ash, cinders, and superheated steam. Accurate estimates of the 
thickness of these flows can only be made after careful mapping on 
account of the extensive faulting, but it probably amounts to several 
thousand feet. 

The first period of faulting.—This period also is not registered in the 
portion mapped, the block-faulting being especially developed in the 
rhyolite flows. The faulting was regular, the blocks tilting in a north- 
easterly direction at an angle of about 20°, the fault scarps representing 
the broken edges facing southwest. The topographic effect of this frac- 
turing is noticeable from any point a few miles south of Tucson, the 
larger mountains (each a fault-block) showing the outline of an asym- 
metrical triangle. 

The intrusions and extrusions of the basalis—Next occurred a number 
of basic intrusions and flows, the history of which is somewhat varied 
in different localities, one phase of which is represented in the Tumamoc 
Hills. Here the first of the basic flows is a basalt, marked B, on the 
map (plate 27), and described under “Petrology” as plagioclase-basalt. 
It appears on the southeastern corner of the map, is well exposed along 
the Nogales road, and is noticeable on account of its large, glistening 
feldspars, some of which have measured up to 2 inches in diameter. The 
broken fragments of these feldspars, set in a dense cryptocrystalline 
groundmass, indicate plainly two outward movements of the mass, in 


76 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. - 


the first of which the magma did not reach the surface, being stored in 
a reservoir long enough to develop the large crystals, and later was 
extruded as a viscous paste which did not flow far. There are a number 
of extrusions of this unusual rock within 50 miles of Tucson, indicating 
that this interrupted and renewed movement is a common phenomenon 
in the later igneous extrusions of the district. The structural relations, 
thickness, etc., are best shown in the sections A~B and G-H. ‘The source 
of the flow was probably south of Sentinel Hill, and it is not unlikely 
hidden under the alluvium in the downfaulted portion southeast of the 
indicated fault-line. The lower portion is somewhat scoriaceous, and 
the original upper surface of the lava was blown to small angular frag- 
ments (less than an inch in diameter) by the stream driven outward by the 
crystallization of the semiviscous mass. There are a few inches of this 
breecia preserved on top of the plagioclase-basalt by the next lava flow. 
Following closely, the intrusion of the neck B, and the correlated 
flow B, occurred, if my interpretations are correct. That the interval 
was short is shown by the accumulation of 80 feet of the plagioclase- 
basalt breccia on the top of B,, which was washed down from the surface 
of the plagioclase-basalt now covered by alluvium. Had this interval 
been long, this fragmented material would have been swept off from its 
exposed position on top of the plagioclase-basalt flow. The neck B, 
(the second variety of amygdaloidal basalt microscopically described) 
shows marked variation in both structure and composition. In the 
center and along the northern edge of the exposure it is coarsely crys- 
talline, assuming an andesite structure. In other places it is a very dense, 
heavy rock, entirely cryptocrystalline, considerably altered, the alteration 
products indicating the large iron content of the rock. In places the 
neck is brecciated and the fragments cemented with jasper, and again 
it contains masses of inflated scoriaceous material which decomposes 
into a red or yellow clay. The flow from the above center is the black 
basalt (B,) which covers a considerable portion of the map. It is the 
most basic rock of the sheet, containing olivine visible to the naked eye 
as brilliant red specks formed by alteration. It is classified as an olivine 
basalt. The structural relations are best shown in the section C—D. 
Following this flow there was a short pause in the volcanic activities 
and the neck suffered some erosion, and then the conglomerate was 
deposited (marked “older wash” and shown south of Tumamoc Hill). 
It is 100 feet in maximum thickness and is overlaid conformably by a 
thyolite-tuff 50 to 100 feet thick. At the time of its deposition the con- 
glomerate was the eastern edge of the Tucson slope, and where it was 
thickest it represents the bed of a temporary stream. Later it was faulted 
up to its present position. Its composition and structure reveal its torren- 
tial origin. It consists of beds of andesite and occasional basalt pebbles, 
in a matrix of fine andesite sand. In size the material ranges from small 


ENVIRONMENTAL AND HISTORICAL FACTORS. TT 


grains to bowlders 2 feet in diameter. The stratification is very well 
marked but has a tendency to wedge out in the direction of the strike. 
The individual layers vary in thickness from a small fraction of an inch 
to 5 feet. 

Igneous outbreak again records itself in the rhyolite-tuff mentioned 
above. It overlies the conglomerate south of Tumamoc Hill and the 
breccia in Sentinel Hill. Talus interferes with the determination of the 
extent of the separation of the tuff from the andesite and the basalts by 
the wash conglomerate. It is the most conspicuous formation of Ttumamoc 
and Sentinel Hills, appearing, when viewed from the south, as a white 
band and cap, in marked contrast from the reddish-black lavas (plate 1). 
It is composed entirely of products of explosive action—volcanic dust, 
glass, and pumice, the source of which was some unknown vent to the 
south. Wherever the conditions are favorable for its preservation, the 
Tucson Mountain slope contains similar deposits of tuff, indicating the 
extent of the explosive action. The deposit was largely eolian, and 
only to a slight extent a mud flow; the small size and small amount of 
inclusions, except those of pumiceous character, and the lack of stratifi- 
cation indicate wind as the chief agent of transportation and deposition. 
Very porous pumice occurs in fragments 3 inches in maximum length; 
the other fragments are of plagioclase and olivine basalt less than an 
inch in maximum length. The bottom layers are quite pumiceous, and 
the upper layers south of Tumamoc Hill are largely pure unconsolidated 
pumice. Again, some of the tuff is baked, recemented, and indurated, 
ringing clear under the blow of the hammer. In.color it varies from 
white through gray, drab, pink, and red. The portions of the tuff that 
are moderately indurated make an excellent building stone, light in 
weight and of moderate strength, but unsuitable for foundations, as it 
decomposes rapidly under the attack of moisture and the humic acids, 
doubtless due to the large per cent of alkalies it contains. (See analysis 
in the microscopic descriptions.) South of Tumamoc Hill to feet of 
conglomerate, similar to that underlying the tuff, is exposed and indicates 
a return to the ordinary wash deposition. 

Overlying the upper conglomerate and the tuff is the last of the lava 
flows, B; being similar in composition and appearance to the earlier flow B,. 
The direction of the flow was north and northeast. The platy-structure 
and reversal of dip is especially developed in this last flow, caused by 
a breaking of the first crust and a squeezing up of the viscous lava 
from underneath. The platy-structure is also formed without disturb- 
ance of the crust, developing whenever there has been a second movement 
in a semi-viscous layer. The reversal of dip is shown on the map by the 
sign (/-)and the dip of the formations by the conventional (|). It is 
not easy to determine the source from whence the last flow issued, but 
the considerable development of the platy-structure upon the top of 


78 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


Tumamoc Hill, the reversal of dip toward its center, and the direction 
of the flow toward the north and northeast make it probable that the 
center is hidden under the body of the flow itself below the top of Tumamoc 
Hill. One of the noticeable features of these flows is their viscosity, thus 
differing from the ordinary liquid basaltic extrusions. This is the reason 
for the peculiar character of the final eruption, which was a swelling or 
piling out of viscous lava, and a consequent sealing of the conduit by 
the erupted material. This possibility is shown in section C—D by dotting 
in a theoretical plug. 

The description of the small intrusion marked B’, and described micro- 
scopically under the name of amygdaloidal basalt, has been delayed 
because there are few data to locate it in the history of the region. This 
was intruded into the andesite underlying basalt No. 3, and probably 
did not reach the surface. It will be found in the southeastern corner 
of the map. The only evidence bearing on the time of its intrusion is that 
both B, and B, are bent up around it, while the tuff does not show any 
change in dip. It is therefore probable that the intrusion occurred some 
time between the intrusion of B, and the deposition of the tuff. 

The second jaulting.—B, was extruded over what formed at that time 
a part of the Tucson Mountain slope, dipping at a gentle angle to the north- 
east. The flow extended as far east as the summit of Sentinel Hill (as- 
suming that the upper basalt covering the summit of the peak is a part 
.of this flow.) Subsequently the basalt and the underlying tuff were eroded 
from the area between the two hills. If the alternate hypothesis is cor- 
rect, that this patch of basalt marks a separate center of eruption, the 
lava overflowing to the north over the tuff, for which there is evidence 
somewhat similar to that suggesting a center under Tumamoc Hill, but 
little erosion need be postulated between the eruption of B; and the 
faulting which gave the hills their present form. 

The existence of faulting is discovered by indirect evidence, as I was 
not able to locate the walls of the break at any point. Some of the evi- 
dence bearing on this is the elevation of both the conglomerate and the 
tuff above the level at which they lie to the west; the sharp cliffs that 
separate the basalt and tuff from the underlying andesite on the south 
and east sides of the hill; the cliffs and truncated formations facing the 
Santa Cruz River on the southeast, etc. The fault around the small 
center hill is indicated by the perpendicular cliffs of basalt underlying 
the tuff on the south and west sides of the hill, and by finding a remnant 
of tuff and underlying andesite-conglomerate in the arroyo bottom, from 
which position it has been faulted up as shown in section E-F. 

Some idea of the time that has elapsed since the faulting can be ob- 
tained by an examination of the fringe of talus conglomerate cemented 
by caliche which surrounds the hill. The torrential precipitation aided 
by steep slope and gravity has no trouble in handling the volcanic blocks, 


ENVIRONMENTAL AND HISTORICAL FACTORS. 79 


formed by the disruptive effects of the flow upon the crust, but the 
assistance of slope and gravity withdrawn, the material is immediately 
deposited. I do not think that 50 per cent of the material thus dropped 
has since been removed, except on the slope immediately facing the 
Santa Cruz River. In volume this wash amounts to less than one-fiftieth 
of the material of the hill. It will be seen, therefore, that erosion has 
scarcely touched the hill since it assumed its present form. ‘This recent 
caliche-cemented formation is called later wash to distinguish it from 
the wash formation underlying the tuff. In places it is thoroughly 
cemented with caliche, developed by the leaching of the porous amyg- 
daloidal surface of the lava flows. The pore spaces of the lava were 
originally filled with calcium carbonate and sulphate, which can still be 
found in freshly broken rock. The cement migrates down slope, spread- 
ing out in places and cementing the material of the Tucson Mountain 
slope, which is otherwise free from caliche. 

The Tumamoc wash just described extends out in a thin sheet over- 
lapping the Tucson Mountain wash. ‘The latter is not indicated on the 
map, but covers the andesite everywhere except in spots south and west 
of Tumamoc Hill. It is a subaerial wash deposited under torrential 
conditions. It is excellently stratified. Its material varies from the 
finest sand to bowlders 6 feet in diameter. The fine sand often forms a 
matrix in which the bowlders are embedded. Wind erosion is now rather 
more active than deposition, and in some spots some imperfect examples 
of desert pavement are developing. The wind is also exposing by erosion 
some of the coarse material formerly deposited in the temporary stream- 
channels. I have named these arroyo-trains,’ and the former course of 
the stream can be imperfectly traced by the heaps of this material at 
the surface. 

There is no direct evidence to determine the time in which the events 
just described took place, but the indirect evidence, such as the recency 
of the last faulting, the freshness of the basalt, and the increased length of 
time that has been allotted to the Pleistocene by recent studies suggests, 
as already stated, that the basaltic extrusions took place in the Pleistocene, 
and the andesite and rhyolite were erupted during Tertiary times. 


PETROGRAPHY. 


The rocks of Tumamoc Hill fall naturally into five types or classes, 
which are recognized in the following petrographical descriptions. 
OLIVINE BASALT. 
This is an exceedingly fine-grained, compact rock in which none of 


the mineralogical constituents can be identified without the microscope. 
When fresh it is black or very dark gray, sometimes quite free from cavi- 











1Journal of the proceedings of the Arizona Miners’ Association, 1905-1906, pp. 13-17. 


80 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


ties, but often rather cellular and even scoriaceous in structure. In 
portions which have become altered these cavities have frequently been 
filled with agate, gypsum, or calcite, minerals resulting from the breaking 
down of the complex silicates. Due to the same reactions, perhaps 
aided by arid conditions, the black basalt has in places become irregu- 
larly covered with thin films of white calcareous material. Reddish 
modifications due to oxidation of ferrous silicates are not common, though 
the black mass frequently presents dark, reddish-brown surfaces as the 
result of superficial alteration, aided perhaps by a concentration of basic 
material on the surface. 

Under the microscope the following constituents may be recognized: 
Plagioclase feldspar, pyroxene, olivine, magnetite, an occasional needle of 
apatite, and rather light-brown isotropic material. The feldspar appears 
entirely in the form of lath-shaped crystals frequently arranged in flow- 
lines and crowded against the olivine crystals. They average about 
0.2 mm. in length and a few hundredths of a millimeter in width. The 
olivine appears as isolated rounded grains rarely over 0.5 mm. in diam- 
eter. Only occasionally does it show its characteristic orthorhombic 
crystallization. Its most common decomposition product in this type of 
basalt is red ferritic material. This frequently permeates the whole 
crystal, but is also often found as a halo surrounding it. In the lavas 
underlying the rhyolitic tuff, this mode of alteration has extended to 
_ such an extent as to make the olivine, which is rather abundant here, 
quite conspicuous even to the naked eye. Pyroxene appears as exceed- 
ingly small grains occupying the spaces between the feldspar laths. It 
rarely has developed into crystals of sufficient size to show very charac- 
teristic features. Without polarized light it is sometimes impossible to 
distinguish it from interstitial glass. Magnetite is present as black, 
unaltered crystals scattered throughout the slide. 


PLAGIOCLASE BASALT. 

This type of basalt consists of a dense, dark-gray to black ground- 
mass, in which are scattered numerous and unusually large phenocrysts 
of plagioclase, with an occasional crystal of lustrous black pyroxene. 
The feldspar sometimes appears as faulted and brecciated crystals fre- 
quently reaching a length of 20r3cm. They are quite fresh and the albite 
twinning can often be observed without the microscope. Angular out- 
lines are the most common, but there are specimens in which the crystals 
have been resorbed by the magma yielding rounded forms. The pyroxene 
appears as isolated crystals, usually about 4 mm. in length and separated 
by intervals varying in different specimens from 2 to 5 cm. 

In thin sections the ground mass is found to be made up chiefly of 
feldspar crystals varying in size from microlitic growths in the denser 
varieties to individuals which can easily be seen by a hand lens in the 


ENVIRONMENTAL AND HISTORICAL FACTORS. 81 


coarser types of groundmass; pyroxene seems to occupy a subordinate 
position. Accessories are magnetite, olivine, and apatite. The pheno- 
crysts of pyroxene are fresh, rather dark colored for this mineral, and 
slightly pleochroic. They show high extinction angle and the other 
well-known characteristics of augite. The feldspars are of a rather basic 
type of labradorite, as shown by the extinction angle on the albite twin- 
ning plane. They are filled with black inclusions of the groundmass, 
sometimes radially arranged. Faint zonal extinction has also been noted. 
Olivine, when present, presents the same characteristics as in the olivine 
basalt described above. It is not at all abundant and in some slides is 
absent. For this reason, perhaps, some investigators might prefer to 
classify this type as a basaltic pyroxene andesite. 


AMYGDALOIDAL BASALT. 


Most specimens of this rock are too badly altered to admit of very 
satisfactory study. Megascopically it is a non-porphyritic rock, only 
an occasional crystal of pyroxene being easily recognized in the fresher 
pieces. It varies from yellowish-gray to reddish-brown, colors without 
doubt due to alteration. It contains in places numerous rounded cavi- 
ties which have become more or less filled with siliceous matter in the 
form of banded agate, chalcedony, jasper, and smoky quartz. Fine 
geodes of brilliant quartz crystals have occasionally been observed. The 
cavities frequently have a shell of agate, the interior being either empty 
or filled with calcite and siderite. 

Microscopically the rock consists of a groundmass of andesitic texture 
containing conspicuous crystals of feldspar and pyroxene with very sub- 
ordinate olivine. In some specimens the feldspar and pyroxene are 
quite fresh, but the olivine and more or less of the groundmass seems 
always to have altered to yellowish decomposition products. 

A similar rock occurs in another portion of the field. It is rather finer 
grained, although quite variable in different portions of the same mass, 
dark brown in color and, like the type just described, has given rise to 
considerable secondary silica in the form of red jasper and agate. Some 
parts of the mass show quite conspicuous feldspar phenocrysts close 
together and evenly arranged. Other specimens show porphyritically 
only reddish and yellowish alteration products of the ferro-magnesian 
minerals. The main mass, however, is quite dense and of even texture. 


RHYOLITIC TUFF. 


This is a light-gray rock, originally consisting of volcanic ash, but now 
consolidated into a mass of uniform texture and of sufficient strength to 
be used extensively as a building material. It contains numerous inclu- 
sions, mostly in the form of pumiceous material but sometimes of darker, 
more basic fragments which have given rise to concentric rings of striking 
appearance. 


82 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


Under the microscope the rock is found to consist of isotropic masses 
of glass and kaolinized matter, numerous quartz fragments, broken feld- 
spar crystals, and shreds of ferro-magnesian minerals. 

The chemical composition of this rock is shown by the following 


analysis: 


Silica (SiOg i: cee ees, rae Aealeraes auc 72550 
Iron (Fe,O,) and Alumina (Al,O3)...... 13.95 
Lime (CaQie teat “eG te. series cote ei: I.4I 
Magnesia (MgO)5..yr Be es nie se 
Alkalies (Na.O and KO) e203 +s oer - 1ie7% 

otalin 5. aa se Ni aiee oe 100.93 


The large percentage of alkalies doubtless accounts for the ease with 
which this rock disintegrates in damp foundations and similar places. 


BIOTITE ANDESITE. 


This rock appears to be made up of a light-gray groundmass containing 
small and evenly distributed phenocrysts of triclinic feldspar and varying 
amounts of biotite. It has a rather pleasing mottled appearance and 
is much used as a building stone in foundations. The chief variations 
noted are with reference to the amount and kind of the ferro-magnesian 
constituent. This is predominantly biotite. In some localities, however, 
specimens have been found in which it consists wholly of hornblende. 
Intermediate varieties also occur. Phenocrysts of quartz occasionally 
come in, making transition types toward the rhyolites. These variations 
may be termed dacites. The groundmass shows very slight variation 
in texture in different portions of the mass, but sometimes exhibits green- 
ish and reddish tints. 

The polarization microscope shows the rock to be a rather acid type 
of andesite. Biotite is in small crystals and in most slides not very 
abundant. ‘The plagioclase, as proved by the extinction angles, are rather 
acid labradorites. The groundmass is made up mostly of feldspar in 
approximately equidimensional masses, giving an appearance somewhat 
resembling the well-known granophyric structure of the rhyolites. Quartz, 
however, in most slides, is not present in determinable quantities. 






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GEOLOGICAL MAP 


TUMAMOC HILLS AND VICINITY 
TUCSON, ARIZONA 


SCALE: 6000 


EEG EINE 





Basalt No. | Basalt No. 5 

Basalt No. 2 Andesite 

Basalt No. 3 Tuff 

Basalt No. 4 Older Wash ‘ 








Recent Wash (a Alluvium 
Probable Fault 













; 2659 = 
LABORATORY 


qlee 
















JECTION E-F 













\ wy 
So 
Sentinel Hil! 















itz, 


OD 









































Tolman, Jr., April, 1908 | 


AMOEN S CO BALTIMORE MD 








THE SOILS OF THE DESERT LABORATORY DOMAIN.? 





INTRODUCTION. 


The present study of the main soil types of the Desert Laboratory 
domain is to be considered as only preliminary, but may be of value 
pending the accumulation of data for a more thorough treatment of the 
subject. In this paper will be presented a brief description of the four 
most distinct types of soils, together with data on their water capacity, 
and some discussion of the moisture conditions which normally obtain 
in them. Data regarding the fluctuations in water-content in these soils 
will be given for the period from October 3, 1907, to April 11, 1908. 

The importance of soil conditions to plant development, a subject 
which has received but slight attention in a scientific sense, even from 
students of distribution, is hardly to be overestimated. It is from the 
soil that terrestrial plants derive their water-supply as well as their supply 
of mineral salts, and therefore the root-system of the plant is perhaps 
more fundamentally important in determining the vital activities of the 
latter than the better known, because more thoroughly studied, subaerial 
portions. The normal functioning of the root-systems, as far as fur- 
nishing water and salts to the entire plant is concerned, is directly depend- 
ent upon the-saline and water contents of the soil. The development 
of a normal root-system, and hence its very existence, depends not only 
upon these conditions, but also upon the supply of oxygen which is main- 
tained in the soil; the oxygen of root-respiration must be derived mainly 
from the substratum in which the roots lie, and not to any marked extent 
by diffusion through the plant-body from the air above. The permea- 
bility of a soil to air is dependent, in a general way, upon properties of 
the soil itself, e. g., upon its porosity or size of particles, but is probably 
dependent even more definitely, in many instances at least, upon its 
water-content. This consideration is of peculiar importance in arid 
regions, where many native plants appear to require a well aerated soil 
for their normal development, and to become unhealthy if the moisture 
content rises too high. The distribution of plant forms is perhaps more 
often determined by availability of oxygen than by that of water. This 
is-a subject, however, in regard to which we have at present almost no 
- definite knowledge. 

In certain rather restricted areas, especially in the arid regions, the 
character of the vegetational cover is dependent upon the soluble content 
of the soil, in a manner quite different from that mentioned above. In 
these cases (of ‘‘alkali” soils) the available water-content is often deter- 
mined, not by the physical nature of the soil itself, but by that of the 

1This section, pages 83-94, was prepared by request and contributed by Dr. B. E. 


Livingston, Member of Staff of Desert Laboratory. 
83 


S86 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


As has been pointed out in Publication No. 50 of the Carnegie Insti- 
tution of Washington, the retaining power of the hill soil is high, being 
48.1 per cent of its dry weight.! The high moisture retaining power 
greatly retards percolation and also evaporation, as well as hindering to 
some extent the absorption of soil-water by plant-roots. The high evap- 
orating power of the air in the dry seasons at Tucson also tends to prevent 
water-loss from the deeper soil-layers. This is explained by the fact that, 
at the beginning of a dry period, evaporation removes moisture from the 
surface layers of the soil more rapidly than this can be supplied from 
below, the evaporation rate being much higher at the soil surface than 
the maximum rate of diffusion through the soil. Therefore, if a dry period 
follows after a rain, the surface of the soil quickly becomes desiccated and 
operates in the manner of a dry mulch to lessen the rate of water-loss 
from the deeper layers. The dry surface layer at length becomes thick 
enough practically to prevent further water-loss by evaporation. At 
the end of the spring dry season, the driest period of the year, the soil 
of the hill has been found to contain, at a depth of from 30 to 4o cm., a 
water-supply amply sufficient for the normal growthof the more xerophilous 
forms of plant life. The actual amount of soil-moisture contained at 
such depth, in July, 1904, was as much as 17.83 per cent of the dry weight 
of the soil; just preceding the advent of the summer rain in July, 1907, 
a sample of this soil from a depth of 40 cm. was found to contain 15.8 
per cent of its dry weight. At the same time, a sample from a depth 
of 15 cm. had a moisture content of only 9.1 per cent of its dry weight. 
At this season of the year the moisture-content of the deeper soils of the 
hill usually equals or surpasses the data just given. 

(2) The soil of the Larrea slope, which surrounds the hill, is even more 
shallow than that of the hill itself. It is underlaid almost continuously 
by caliche, which is usually found at a depth of 30 cm. or less. Above 
the caliche often occurs a layer, some 10 cm. in thickness, of coarse gravel 
and rock fragments mingled with comparatively little soil. The soil 
above this is more loamy than that of the hill, with an admixture, usually 
approaching half its volume, of finely fragmented lava and caliche. The 
soil here is thus seen to be exceedingly well drained; with its under- 
lying caliche, which is nearly impervious, it is unable to hold nearly so 
much water as does the adobe clay of the hill. Beneath the caliche 
there are occasional deposits of soil, but evidence has not been found that 
any considerable amount of precipitation water often penetrates to these. 

This soil has little or no tendency to crack when dried out from the 
wet condition, partly because of the lower moisture-holding power, and 
perhaps also in part because of the larger proportion of small stones. 
A sample of this soil showed its moisture-holding power to be 20.1 per 





' All samples used in deriving the soil-moisture data here to be presented, as well 
as for data of Publication No. 50, were unsifted, but no pebbles larger than 8 mm. in 
diameter were included. : 


ENVIRONMENTAL AND HISTORICAL FACTORS. S87 


cent of its dry weight.’ This factor, together with its shallowness and 
loose physical structure, allows rapid percolation and rapid evaporation 
after rain. It appears that after saturation has been attained a con- 
siderable amount of water is drained away along the caliche surfaces 
below. Thus it appears that almost the entire mass of this soil is nearly 
air-dry during the greater portion of the dry season. This dry condi- 
tion probably accounts for the absence here of almost all vegetation 
excepting the characteristic Larrea and a number of forms which are 
active only in the rainy seasons. Larrea appears to obtain its moisture 
from the small amount in the soil and perhaps from the still more meager 
supply in the crevices of the caliche. 

Attention should be called here to the fact that the soil of the Larrea 
slope receives more water than does that of the hill. The former is 
exposed at a lower angle, so that precipitation water does not flow off 
as rapidly as it does in many situations on the hill, and it is often flooded 
after a rain by the superficial run-off from the hill itself. Any advan- 
tage accruing from these conditions, however, is apparently more than 
counterbalanced by the shallowness of the slope soil and its drainage 
facilities, as well as by the ease with which it becomes desiccated. 

(3) The soils of the wash are sandy, varying from a coarse gravelly 
sand, in the present intermittent stream-channels, to a light loamy sand, 
in areas not flooded at all or flooded only when the wash is running full. 
The stream channels are somewhat lower than the loamy areas, and 
the latter have the character of miniature flood-plains. Practically no 
vegetation occurs on the coarse sand, these soils being exceedingly well 
drained and almost constantly dry to a depth of many centimeters. 
Their surface layers are indeed moist only for a few hours following the 
disappearance of flowing water in the wash, which is present only after 
the heavier showers. A light shower appears to have very little perma- 
nent effect in raising the moisture-content of this soil. 

The comparatively luxuriant vegetation of the wash is rooted in the 
more loamy deposits, and it is here, and on the river flood-plain, that 
the most mesophilous plant-forms of the region occur. ‘These soils are 
30 cm. or more in depth, usually overlying coarser sands. The loamy 
deposits have a water capacity of about 25 per cent of the dry weight 
of the soil, a capacity which is seen to be somewhat higher than that of 
the soils of the Larrea slope, notwithstanding the fact that the wash soil 
is much more sandy than the other. This difference is probably due to 
the absence of a large amount of broken-stone in the former soil and to the 
almost complete absence of such material in the wash. 

The loamy sands of the wash receive much more water than either 
type already discussed; the flow of the wash includes, besides direct 
precipitation, the run-off from a large drainage area, so that they become 





*Had the pebbles normally present in this soil been sifted out, of course the moisture- 
holding power would have been higher. 


88 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


well moistened with each period of high water. The water readily pene- 
trates to great depth, and appears to be fairly well held, at least in the 
deeper layers. It is probable, although no direct evidence is at hand, 
that these soils possess a subterranean water-table, at least during many 
months of the year, and it seems not unlikely that the larger perennials 
may obtain moisture from this source. 

(4) The soil of the river flood-plain is of unknown depth, without caliche 
or other impervious hardpan. At the time of the digging of the well for 
the Laboratory water-supply, an opportunity was presented to observe the 
conditions of this soil from the surface layers to depth of permanent water. 

The surface layers are composed of a clay loam which possesses a 
water-holding power of 38.5 per cent of its dry weight. At the well site 
this loamextends to adepth of about 5 metersand then gives place to strati- 
fied sands and gravels, with some water-worn bowlders at a depth of about 
g meters. Permanent water, in coarse sand, was met at a depth of about 
12meters. Nearer the river channel subterranean water occurs ata less depth. 

At the time the well was dug, March, 1906, a number of determina- 
tions of the water-content of this soil were made. Samples were taken 
in closed bottles, weighed, dried in an oven at a temperature of from 
105° to 110° C. Table 3 gives the moisture condition of the soil at this 
time in terms of percentage calculated on the dry weight of the soil. 


TABLE 3.—Morsture Conditions of River Plain, March, 1906. 








Nature Water 
bs atare of soil. content. 
Meters. Peet. 

0.20 Loam. £250 
3.30 Loam. 19.0 
4.00 Loam. 2250 
5.10 Loam. ee | 
Sues Sand. rae 

















From table 3 it is apparent that the water-content increased from 
the surface downward to the limit of the loam, and that in the sand the 
water-content was low, too low to be of much benefit to plants, the roots 
of which might penetrate to such a depth. No determinations were 
made in the vicinity of the water-table, but it may safely be assumed 
that for some distance (perhaps a meter or two) above this the sand 
usually contains a considerable amount of water. 

The relatively great depth of the water-table makes it appear that the 
general vegetation of the flood-plain derives little, if any, of its water- 
supply from this source. Some trees here, especially the native mesquite 
(Prosopis velutina), may send roots deeply enough into the soil to reach 
layers kept permanently moist from the underground flow, but the natural 
herbaceous growth on the plain probably derives all of its water-supply 


ENVIRONMENTAL AND HISTORICAL FACTORS. S89 


from precipitation water held in the deep surface layer of highly reten- 
tive clay loam. 

Formerly the river spread over this plain in times of flood and made 
of it a marshy area or cienega, but deep gullying of the stream-channel 
has lowered its bed several meters and rendered the present flood-plain, 
where not irrigated, a parched and barren waste in the dry seasons. In 
uncleared land occur numerous low trees of mesquite, catclaw (Acacia 
greggut), etc., but, as stated above, these may be able to obtain water 
from deeper sources. Where irrigation water is conducted to it this plain 
is exceedingly fertile, and in the rainy seasons it supports, without irri- 
gation, a luxuriant growth of those herbaceous plants which are active 
only at these times. 3 

Evaporation from this soil, while much more rapid than from the soils 
of the hill, is considerably retarded, and here also the dry mulch is notice- 
ably effective. The fact that the surface is nearly horizontal, so that 
precipitation water does not rapidly run off, gives to this soil type a some- 
what larger water-supply from precipitation than that received by the 
other types here considered. 


THE MARCH OF THE SOIL-MOISTURE CONTENT. 


The prime importance of the moisture-content of the soil, in determining 
the behavior of plants, makes it a matter of keen disappointment that no 
very satisfactory method for measuring this function is available. An 
instrument which should give data on the amount of soil-moisture at a 
given location and depth, without the necessity of disturbing the soil, would 
be as great a boon to agriculturists as to students of plant distribution, 
but, unhappily, no instrument of proved reliability is yet available. As 
far as I am aware, only two instruments have been suggested for the 
determination of the moisture of the soil 7 sztu, and with both these 
there are practical difficulties of operation, or interpretation of results, 
which prevent their being wholly satisfactory for field observation.’ In 
the absence of any better, the commonly used, but very crude method of 
sampling and drying was resorted to. 

Beginning on October 3, 1907, and continuing till April 11, 1908, a series 
of soil samples was taken, at intervals of about 10 days, from four stations 
representing the four soil types with which we have to deal. These were 
dug with pick and trowel and collected in tightly stoppered bottles of 
known weight. The data of moisture-content for these samples were 
then determined in the ordinary way, by weighing, drying in the oven, 
_ (temperature 105° to 110° C.), and reweighing. All samples were taken 





1For these two methods see N. Y. State Agric. Exp. Sta., Ann. Rep., 4: 176-179, 
1886, and Briggs, L. J., Electrical instruments for determining the moisture, tempera- 
ture, and soluble-salt content of soils, U. S. Dept. Agric., Div. Soils, Bull. 15, 1899. 
Mention should also be made of bulletins 45 and 50 of the Bureau of Soils, which are 
occupied chiefly with the determination of the moisture equivalents, or optimum 
water-contents of the soil. 


90 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


at two depths—for the hill, wash, and river-plain at 15 and 30 cm., and 
for the Larrea slope at 10 and 20 cm. ‘The localities of the four stations 
are shown on the map, plate 26, where the hill station is marked I, that 
for the Larrea slope II, that for the wash III, and that for the river-plain 
IV. All samples for the same station were taken within the same area 
of a few square meters, the small excavations made in obtaining them 
being immediately refilled, and no later sample being taken within 50 cm. 
of the soil thus disturbed. On December 13 it was found necessary, 
on account of irrigation water, to remove the station for the river-plain 
from the position marked IV, on the map to that marked IV,. Otherwise 
the stations remained fixed throughout the period. The deeper sample 
in the wash was considerably more sandy than the shallower one. After 
December 3, 1907, the samplings and moisture determinations were 
performed by Mr. J. C. Blumer, to whose careful work the series of obser- 
vations is fundamentally due. 

Records for Tucson, extending over a period of 15 years (Publication 
No. 6 of the Carnegie Institution of Washington, pp. 26-27, or Publication 
No. 50, p. 23) show a mean annual precipitation of 30.10 cm. (11.74 in.), 
occurring mainly in two rainy seasons. The mean monthly precipita- 
tions, together with the actual precipitations for the months from July, 
1907, to March, 1908, are given in table 4. 


TABLE 4.—Mean Monthly Precipitations at Tucson, from Records of 15 years, and observed 
Precipitations for the Months, July, 1907, to March, 1908. 











Month. Mean. 1907. Month. Mean. 1908. 
cm. in. cm. mM. cm. nN. cm. mM. 
July. 2. se ch Oe15.| 2.40411, 9.54267 My January: oe) 625030100 790 ia ban ee oo 
AUZUSts. . 6 6.67 | 2.60 | 9.32 | 4.67 || February...) 2.31 | 0.90] 4,78 \41..90 
Septembers 2067 f Gic t64G 294. | toe Marcht@ no ©. 977) 02777) 1-359 os 
October 2... wit «64 OLG4 ee. AL 0505 eh Dill eae @:60 WOSe7 aS 2 Crises 
November. -] 2.08 |0.81 171-88") @.74 || May O 26500 214 Ale ieee nee 
Decemberay) h2556" (05300 5\0'.001) O00 i Juneraa tee, O867> 110 326 ieee. t iene 






































The summer rains began on July 3, 1907, with a shower amounting 
to 0.08 cm. (0.03 in.). The first effective shower, however, occurred 
on July 7, at which time 1.52 cm. (0.60 in.) of rain fell. After this the 
rains were well distributed throughout the remainder of July and August. 
It is apparent from the table that, until February, the period of the soil 
observations was exceedingly dry, although of course not as dry as is the 
ustial spring and early summer. Before the beginning of the soil records, 
the last effective shower occurred on September 3, amounting to 0.53 cm. 
(0.21 in.), and on September 17 occurred a shower of 0.12 cc. (0.07 in.). 

The rain-gage is located only a few meters from the station for soil 
observations on the hill, and there is no question as to the applicability 
of its records to the soil data for that station. Owing to the local nature 
of many showers in this region, it is possible that the precipitation recorded 


ENVIRONMENTAL AND HISTORICAL FACTORS. Q] 


here is not exactly that of the other three stations, but the discrepancy 
can not be great. ‘This local character of the showers is most marked 
in the period of the summer rains, and is not so strikingly manifest in 
the winter season. 

In table 5 are presented the soil-moisture data, in terms of percentage 
calculated on the dry weight of the soil. In the last two columns of the 
table are the precipitation data for the short periods between observations. 


TABLE 5.—Soil Movsture and Precipitation, from October 3, 1907, to April 11, 1908. 






























































a Per cent of soil moisture, calculated on dry weight. a 
Precipita- 
: r . : tion since 
Date. Hill. Larrea slope., Wash. River plain. Fee ee ok 
vation, 
15 cm./30 cm./10 cm. 20 cm.|15 cm. 30 cm./15 cm./30 cm. 

cm. in. 
TQO7, OCEss 1.707 C25 1 Post Noses Sas 3.6 3.5 O83 oe Seer ae oe 
(Cte PA ae Ol ita) Pairs Pi ede ROH) 3% FAM TSO Hi OAO40.04 
Cita ee ewe Gis ee TOs e ae OR el LAR | O.62 
OL20 ap ig rr toe Prost 3.4 187.7 PTAO Vy $662 1 0.04 | 0.25 
Nove To8be er Orr gos: eG On le NAO} 8.5. lost 9.1 Ol dt 520.60 
NOVig2 7 ei oa ue Pe 752 ie Ont 6.7 2.7 etOLd 9.7 0.46.) 0318 
Decree: 18.7 | 17.6 FA 6.3 5.9 ART I TOS Paki, $0.00" O.00 
Dect a2 pai 505 Only SOU WW 92 003-5 GAM O90 18O,001C 0.00 
MB ee oe Seem ies) 8 be! eg Baler. 6:6.) 5.0 4.4 2. Fo TOA 23.7. O00 a 0.00 
19008, [ale 2. 447. Ry Oe Ea 6.4] 5.8 FY Sepp 1L-O' | 7.04 O00 |) 0100 
VARS Ror eer yeaa) 7k Sia i OT te ALO Aa 2 O55 2182857 10262 
ete elie De Oap Ler 6.9. 9,6.7 O76) 5:5.) 1O:7 SO O.12. 10.05 
Hepes rT)... TS.0 1145 ood | 8.2 Sea | A.2 |) 10:4 8.4) O00} 0,00 
Pell Ser sa Oe Ori) DIRGIRES. GAN TAO  terd-Oll26.07)/ 27.54 2278) 1.49 
Bela? Gee ae ree Pee 1452 110.7.) 8-2 1 10.0 } 16.0 10.614)50,.24 
Mares soa 27-2 NTs 9.6 | 10.9 9 he C2 °F 19.0 VEO. 0:38 Ors 
Mare foray 025i ueeorr 7.9 9.5 6.1 ATM, DAIO Ws T5248 O25 |) hO211 
Mare 22 22 10.0) m1o.s 6.9 725 5.8 4:6 $17.0.) FOs1 || 0.07 0.38 
Petit Leas ercnoey SG o3 PT LOly 5.8 | 6.8 5.1 Woe Fie. 34 26.3 | O10") 0104 
April zi hitsoF3 8.1 SATS) He6 5.3 it. OUSTO.9- | T1264) O.001) 0°60 

1For o to 10 em. depth. 3For o to 15 cm. depth. 

*For o to g cm. depth. 4For o to 20 cm. depth. 


®10 cm.(4 in.)of snow fell on this date, and the record may below. The snow re- 
mained but a few hours. 


It is unfortunate that the soil observations could not be begun before 
the end of the summer rains, at which season the soils approached their 
maximum water-content. The series of data here given is, however, 
instructive in regard to the effectiveness of the precipitation in moisten- 
ing the soil, as well as in regard to the degree of water retention exhib- 
ited by the several soil-types. 

All of the data given in tables 4 and 5 are presented graphically on 
plates 28 to 31. In the curves of soil-moisture there presented, the 
abscissas represent time and the ordinates the percentage of soil-moisture. 
Each plate represents the conditions for one type of soil. The curve 
for the greater depth is, in each case, drawn as a full line, that for the 
lesser depth as a broken one. The numbers on the curves denote the 


92 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


percentage (ordinates) and are taken directly from table 5. The pre- 
cipitation has also been plotted, in the form of a gradatory graph, the 
vertical lines denoting the amounts of precipitation occurring on the 
corresponding dates. The figures above the graph show the amount of 
precipitation recorded. Traces are denoted by a hachure above the 
graph, accompanied by the letter 7. The horizontal lines serve merely 
to guide the eye. Hachures and figures below the graph denote the 
dates on which soil-moisture determinations were made. This notation 
facilitates comparison of the precipitation graph with the curve of soil- 
moisture. To further facilitate this comparison, the precipitation graph is 
repeated with each curve of soil-moisture, it being assumed to be identical 
for the several stations. Vertical lines through the curves denote the 
position of every tenth day, beginning with October 1. 

In attempting to interpret these curves of soil-moisture it must be 
remembered that the samples for the different dates are not from exactly 
the same point in the soil, and that where local variations in the mois- 
ture-content occur these may lead to unexplainable irregularities in the 
curves. This consideration is especially important in the case of the 
hill soil, where the irregularities of the underlying rock-surface, as well 
as the presence of large rock-masses in the soil, cause marked variations 
in soil-moisture at any given time. Masses of rock which lie partly in 
the air and partly in the soil are effective in the control of the soil- 
moisture about them in two ways: they facilitate the downward pene- 
tration of rain-water along their surfaces, and they greatly hinder the 
loss of water which has once penetrated to soil which lies beneath them. 
This disturbing effect of local variations in soil-moisture is of less impor- 
tance when the question at issue is the average moisture-content of the 
soil for a period of several observations than when the fluctuations in 
soil-moisture are to be considered. But at best, the method here used 
is to be regarded as giving results which are only approximate; its prob- 
able error must be relatively very large, although we have at present no 
means of determining its magnitude. 

From the curves it is apparent that the moisture-content at the lesser 
depth varies from time to time much more markedly than does that at 
the greater depth. This of course is to be expected, for the surface soils 
are first to be affected by precipitation as well as by evaporation. The 
deeper samplings, however, are more valuable than the others in relat- 
ing soil-moisture to plant activity, for most plants of this region gain 
access to a depth of 30 cm. in a very short time after germination, and 
the soil-moisture at this depth is probably a fair measure of the water 
available for the general activity of most forms. The cacti are, in general, 
a marked exception to this statement, as are, indeed, many other forms 
provided with storage organs. 

All of the curves exhibit also the very important lagging of the effect 
of precipitation behind the march of the precipitation itself. Thus, it 


ENVIRONMENTAL AND HISTORICAL FACTORS. 93 


may be a number of days after the occurrence of a heavy shower or series 
of showers, before any effect of these is felt at the depth of 30 cm. This 
consideration emphasizes the inadequacy of mere precipitation data in 
any attempt to determine the moisture conditions under which the plants 
of any region live. The lagging in the opposite direction is also well 
shown by the curves, and is of even greater importance to plant-life in 
the desert. Thus, a dry period of several weeks may ensue without the 
occurrence of any appreciable change in the moisture conditions of the 
deeper soil layers. In general, the heavier the soil, the more important 
is this principle, but other factors must be considered. In the case of 
the hiil soil, the water contained in the deeper portions is lost only through 
plant transpiration and direct evaporation at the upper surface, but in 
that of the soil of the river-plain water is lost from a depth of 30 cm. 
both by an upward and by a downward movement. The underlying rock 
of the hill prevents water-loss downward, while the dry underlying soil- 
mass of the flood-plain absorbs water with great avidity and thus acts 
not unlike a process of evaporation effective from below, as in the case 
of a suspended porous clay pot of soil. 

From very limited data’ on the relation of the activities of desert 
plants to soil-moisture, it appears that the minimum for the activity of 
forms not possessed of storage organs probably lies in the vicinity of 10 
per cent as here calculated. Making such an assumption, it appears 
that the hill soil at a depth of 30 cm. was in good condition for plant 
activity throughout the period of observation, and that the soil of the 
river-plain at the same depth was not in as good condition. The con- 
ditions at a depth of 30 cm. in the wash and at a depth of 20 cm. on the 
Larrea slope can not be regarded as favorable to plant growth, although 
during February and early March considerable moisture might have been 
available here. 

The moisture conditions of the surface layers of the soil must be re- . 
garded as the prime factor in the determination of germination, since 
seeds seldom find their way to very great depths and are unable, in most 
instances at least, to germinate without a relatively great supply of 
oxygen. It appears from the curves that germination was probably 
possible in the surface layers of all the soils throughout February and 
early March. In order that most perennials may succeed in the desert, 
their seeds must be able to germinate in the surface soil when this is 
moist, and the seedlings must send their roots downward rapidly enough 
to reach the deeper soil layers before they are overtaken by the desicca- 
tion of the surface soil. A large proportion of the seeds which germinate 
in the vicinity of the Desert Laboratory meet an untimely end through 
failure to fulfil this condition. 





1For some data on the minimum of soil-moisture with which plants can be active, 
see Publication No. 50, Carnegie Institution of Washington, pp. 66-67. 


PLATE 28. 





3 an 20 30 10 2030 20 30 10 20 ete 20 i Feaius 
OCTOBER,I907} NOVEMBER herrerd: JANUARY,1908] FEBRUARY Hee APRIL 





3 1418 26 Lis 23 13 13 eT eo 12 TT 
OCTOBER,1907+ NOVEMBER ipeseeee eee FEBRUARY ne agile 


CURVES oF Sori-MorsturE, Tumamoc Hu. 


PEATE, 29. 


. wack 
3 ZOnSO (Os eZO eee lOREZO aso [O27 20 33:0 lO Zo 2030 10 
ae 1907 eee | DECEMBER JJANUARY,1908] FEBRUARY eee APRIL 











- CURVES OF SOIL-MOISTURE, Larrea SLOPE. 


PEATE 30. 



























10 24:20 
—s 

BHO 1207 30 lO ZOrs@ 20 30 

OCTOBER,I907} NOVEMBER MARCH 





[OS ZO RS) LOMRZO SO 10) "20 10 
DECEMBER |JJANUARY,1908] FEBRUARY APRIL 





3 1418 26 i323 13 i3 23 {2 13 27 |I a ee eerie i 
OCTOBER,1907} NOVEMBER DECEMBER |JANUARY,1908] FEBRUARY MARCH APRIL 


CURVES OF SOIL-MOISTURE, WASH. 





PEATIESaa 


20 30 10 20 30 1020030 AO pe20 ive 
OCTOBER,|!907 NOVEMBER DECEMBER MARCH APRIL 





ayr4 1B. 26 Rots Tala oe ie ee ee nan: STi he ON I SEN Ili 6 23 peels 
|OCTOBER,I907} NOVEMBER | DECEMBER |JANUARY,1908] FEBRUARY MARCH APRIL 


CURVES OF SOIL-MOISTURE, SANTA CRUZ PLAIN. 


94 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


CLIMATIC CONDITIONS. 


The general climatic conditions which prevail in the region in which 
the Desert Laboratory is located are well known. They are, briefly 
stated, such as are necessarily characteristic of more or less elevated, 
semi-arid, continental areas in lower and middle latitudes. The annual 
precipitation is meager and irregular in amount, and this, with low rela- 
tive humidity, high rate of evaporation, wide diurnal range of tempera- 
ture, high winds, and intense insolation, presents a combination of trying 
conditions, to which, however, a large number of plant species have 
successfully adapted themselves. 

These conditions are, in general, characteristic of all the great desert 
regions of the globe, which, however, differ widely in important partic- 
ulars, the differences being reflected more or less distinctly in the habits 
and distribution of the plants inhabiting them. As an example, the 
distribution of rainfall in the desert region of the southwestern United 
States is essentially different from what it is in the Egyptian-Arabian 
deserts. In the latter the period of maximum rainfall and of greatest 
vegetative activity is in the winter, while in summer the vegetation is 
practically dormant, the time at which on the Laboratory domain the 
summer annuals are at the height of their vigorous growth. Such a 
state of affairs necessarily results in great differences, not only in the 
character of the summer vegetation, but also in periods of growth of 
perennials, and in structural and physiological relations. The studies of 
this subject at the Desert Laboratory by Cannon (1905) are highly sug- 
gestive and show the desirability of extended investigation. 

The lack in botanical literature of sufficiently detailed observation and 
experiment regarding local climatic conditions in their relation to the 
growth and distribution of plants suggests that, without attempting a 
discussion of the broad subject of desert climates, it will be desirable in 
this place to call attention to a limited number of recent records and 
studies looking toward more exact knowledge of local conditions and 
their influence ou the distribution of plants. 


RECORDS OF RAINFALL. 


Records of rainfall show that in this region the amount of precipita- 
tion is extremely variable from year to year in the same place, and that 
it often differs very considerably in places not widely remote from each 
other. In table 6 are given the records as taken at the Arizona Agri- 
cultural Experiment Station and at the Desert Laboratory, 3 miles 
distant and at approximately 200 feet greater altitude, for the period 
March, 1906, to May, 1907, inclusive. They show a fairly close corre- 
spondence for most storms, but it is also seen that summer rains espe- 
cially, even at this short distance, are often extremely irregular and 
unlike in amount of precipitation at the two points. As an example, 


ENVIRONMENTAL AND HISTORICAL FACTORS. 95 


the records for July 14-15, 1906, give an aggregate rainfall of 1.16 inches 
at the Desert Laboratory and 0.49 inch at the Experiment Station, 
while for August 10-11, 1906, the amounts recorded are a trace at the 
Laboratory and 0.88 inch at the Experiment Station. Enough is known 
of the habits of the plants of the Laboratory domain to establish the 
fact that such differences of precipitation profoundly affect development, 
and in critical years may be associated with conspicuous failure on the 
part of many species of annuals to appear in their customary places. 


TABLE 6.—Records of Rainfall at Desert Laboratory and University of Arizona, 
March, 1906, to May, 1907, inclusive. 



























































Date. Lab. | Univ. Date. Lab. | Univ. Date. Labs 4 Untv; 
1906. Inches. | Inches. 1906. Inches. | Inches. 1907. Inches. | Inches. 
Mis ratte Os OL 75 MPAs ter Uo tei ec Tat MA BSiO .0O2%) fa 202 
eo | Cee “E3 Ince O-O0Le | O00 Cieiae coats 1% 
Doe Ren. fie Fis datire ey: . 42 ee fo See Ae 
Pe. Oi. 0.03 Por) O47 0.59 5 RA Oe ee O.O1 
ZOnn O.. 23 0.26 TO Oe sO) HLOs2s ES fa LO ex 
ZT et O.0225) 0.045 COM Oar | Orse 16-2) 0.02 }.0.20 
P 8 0Ces engage. ae ; cee cae O.OI Cte) Sater ah: 
mote O- 1-0 .01 |. O-30 ig ii. aera LO Meee eat o.25 
PIANO 34 0.14 BEEN, OOF Ws sts 19.4) O35 Oui! 
May 23) 4) @.0183|"1. One i oe 2OcLNe Lo 0.01 
ty eter 0.02 Brea O.02 1G, OF 2004) On 8F) | 0.20 
tO. O83 50 tr screae: SeEptgn0 yor err fy. Bir ies. Jee O.O1 
Gen peace Pe PA 210) | Osa4 HebuitG. cr 2) 0.035 
LO errr ee 0.04 27 O18 |. 0200 Pore ie O.OI? 
TS, LOMtIVe:, 16 Octo, 01605 141. 2 (ewe SE8e 0.04 
he et eee ee nee NOVA. 1smi.00055 | 0.07 25d 0.08 4. 0.025 
oe. el, aye a aso eee ‘ FW ha cea im! 
1451 GOO WO 440 LOM. OrAUS) | O52 Mag. 95: 210.256 1.0240 
Ih ach 0.27. -|.0,09 2A 2) O219e OnL5 22 el OSLO) On 1G 
TO eee 0.04 SSE DU haste: 218% aad Meds a ec 
Bia) bees, ce O.O1 Decoy 2 io.65a) |:o390 ie tala) Ls) ey teed 
EA eee ‘ie Beles OO tee aT 5 CA ANE 0.015 
2) nt a Velho Mi ees elt DB ly eee es ee i da eae Te 
260150702 1.06 Deir Oper Mata Lt oy perk he ae eee 
ae POROOS | oan AOpen ST 1 Oe 20 este ti Ake ‘be 
i: A pS al AORN Pana 0.03 AS hosed le se ws At 
AT eye ihe 1907. Mayon oto jee 
Aum erie OsrTeo to. 3r SY ioe ie aah Bey et Pho ad cesar 0.06 
les Wee ; Zee O-OF Outs 2Ot| Orton | Ort 
Toa) als 0.26 Che SOLOL= WOe1G 
5% Sie ea 0.62 ide aed 0.08 
1Snow. 


The records of table 6 are also of importance in connection with the 
measurements of the sahuaro, which prove its capacity for the prompt 
absorption and storage of water after even very light rains. ‘There can 
be no doubt that by this means the giant cactus is placed at a distinct 
advantage throughout its range and in its association with more deeply 
rooted plants. 

More definite, as regards the intimate relation existing between dis- 
tribution of precipitation and of vegetation at various points in southern 
Arizona, is a recent paper by Prof. J. J. Thornber to which I am per- 
mitted to refer in advance of its publication elsewhere. A comparison 


96 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


of winter and summer rainfall at 53 different places, from such records as 
are accessible, shows that, in general, at points east and south of Tucson, 
the aggregate summer rains are considerably in excess of the winter 
rains, while to the west of that place the reverse is true (table 7). This 
difference is due to the fact that to the eastward and southeast the Gulf 
storms become more and more a controlling element, while on approach- 
ing the Pacific the conditions become dominant which, in southern 
California, limit the rainfall to the winter months. This difference is 
such as to very noticeably affect vegetation. Particularly is this true of 
range grasses which by reason of preponderance of summer rains to the 
east and south find there more favorable conditions for development than 
to the westward, a matter of much economical importance to stockmen. 

Table 7, compiled by Professor Thornber, shows how few are the excep- 
tions to the general rule as stated, and that it holds true notwithstanding 
rather wide differences of altitude, which necessarily affect the amount 
of rainfall. 

TEMPERATURE RECORDS. 


The account that has been given (pp. 44-47) of soil and air tempera- 
tures at the gulch near the Desert Laboratory points to differences of 
temperature as the primary factor to which the remarkable differences 
of the plant covering on the two sides of the gulch are due. In this case 
it has been possible to establish a causal relation between observed facts 
of distribution and a single physical factor, which though operating in 
part indirectly is none the less the fundamental cause of the phenomena 
in question. The rather voluminous readings from which the results 
are deduced are given in tables 8 to Io, in sufficient number to indicate 
the nature of the data. Maximum and minimum readings are for either 
one or two days, except in a single instance in which a period of three 
days was covered. 

In field studies of this nature nothing is more important than to bear 
in mind the futility of attempting to account for facts of distribution 
without due consideration of all the factors involved. To emphasize 
the influence of a given factor is one thing; to ignore those acting in 
conjunction with it is quite another. As pointed out by Dr. Cannon (p. 60 
et seq.), the superficially placed root-system of Cereus giganteus subjects 
it to conditions of soil temperature widely different from those affecting 
Parkinsoma nucrophylla, with which, however, it is almost constantly 
associated, and innumerable similar cases emphasize the fundamental 
fact that plant associations, while often conspicuously related, as a whole, 
to a single factor, are composed of species often of widely different habits, 
responding in different ways and degrees not to one, but to all the factors 
of their environment. Under such circumstances any practical method 
of simplifying the problem of measuring and correlating physical factors 
with choice of habitat is particularly welcome. Some progress has been 
made in developing such a method, as will be seen on pages 99-100. 


ENVIRONMENTAL AND HISTORICAL FACTORS. 


TABLE 7.—Drstribution of Rainfall. 


STATIONS WEST OF THE MERIDIAN OF THE DESERT LABORATORY. 


YS 





oF 





Station. Pe ee an cain cipitation 

’ Feet. Inches. Inches. Inches. 
PIGCeNCGMEe ga taa ft: cathe. 13 1,700 2.81 Sak 5 9.78 
Casal S teres ec We. wh 14 1,395 ronwae 2.07 5.29 
Manica pawt tts aes 2 3 18 We Ayes 1.97 a1G 5.50 
iMdatalcet s(t ea Sl ree a geen 15 1,100 2236 3. OF 7.08 
NGkenere Tuer. cao. es 10 2,086 3.46 5.89 9.85 
ECL Nste mente fe  iltea 9 RoR 19 150 0.65 2.06 3605 
0 Te PA a oe a 13 4,501 a Pen ee 8.61 17 2 
PECSCOLLER TS Fs ous oe oe 27 5,304 6.89 8.34 16.25 
Fort McDowell....... 23 1,800 2.62 ae: 10.45 
BAS a Oe i Sk One. Serna 5 1,865 oly | 4.87 10.99 
EIEIO EV ALLCVe ia aa cet ee wee arn e § 6.32 12.85 DIALS 
Parley c Campa. ats Ad Mittin tnt Stn i 9.46 Gye 
ING@wy GN tvege 4 ce en Ste Qe ka aes ZL 1 meaty TO. EY 
PUCAWHETE Ye. te ee a oe el teres cr ee 17.20 26.59 
WialnutiGroveds.s oh de: oa. Be eee ee eles 8.60 het 
Wood Canyon 9.0 2.1. os =. So a ee 9.50 ee ew Pe her ay 
Horaibouaye. scoke. sate hh ole wee 600 0.95 3.90 5.32 
Wiles Gali in ery oo elede oe 9 6,870 Ton 12.08 25561 
SERS? LAV RES! orbs. fe, yy ee en) Eee et 0.74 1.91 3.09 
La Me cpr rc eat eee Wc’ ons Ce) ee knee Ax73 G233 12.19 
Stlagmeliiens tte aa cules al 5 as 1295 3.68 6.15 
INeeurak Bridge. 4. Gaus a2) | a ee 5-94 kino 19.49 
Peoria ties.) ee tee Sk Rear rae se wk 2.79 4.68 8752 
PeNIe a sg oe eee ee 5 1,000 Bert 28 8.63 
DES are Sc OMe dete Ek A eh RN tee 8 60 17.07 
MeQOdritit,.. ne dehec ck coe. «2 een 1 eed, ao 7 6.54 | Ore Ye, 
Ne Ras leiy 2.8! eu geee Mee thee MPR eee 8 5,057 3.68 a5a2 8.46 
A SGOHEY, Sp TUN fer ee 28 ale ee as: 8,000 8.93 E704) 30.07 
Canip DalrCreck: (sa... oF ey ae 6.87 6.52 {A422 


























STATIONS EAST OF THE MERIDIAN OF 


THE DESERT LABORATORY. 











Fort Lowell 
San Carlos 
‘Tucson 
Benson 
Pantano 
Baipire hauch: jee. oa dee ys 
San Simon 
Weenie mame Se ee ee 
Fort Huachuca... . 
Fort Grant 
Pcie OWI. ee es ean 
ote Denhance... 26 cane 


Cy ety eee et Pe ied 
eal eae 16) een ine gien sid’ omaha, 5: 
SP vat efaist ope) (G0 Wene) er se) he, a: iaui@’ i ai Ae 


a) a) Ke oe AO Sacer 
ae, 6 |e: ele) yapetel is! vel" [oj /a'e) (6), 


ost. Ske tet se. aiisy 6. el 8t a) 6. 0 


Calabasas 
Camp: Crittenden: — oh. estan. - 
Chiricahua Mountains...... 
EW aS OOM a taps nic tiny Areas 
Dragoon Summit... eee 
Lochiel 
Walout Ranch: is. evowa ees. 
Piatt DAMICCe 5 so ccss ok «er es 
Fort Buchanan 
CRON a eee sb ag 


at > bie) ate w, Be, vB (6) 6) 8) fa! lis 


os we Roe. eo Lela lence 








BAWOMNWHWANUYW 





2,539 
2,400 


2,371 


<3) a) te) © (By ie) efile 


6,6. 0) et de fast 


CC CRMC) Te] 
ef Sh ay 0! (ein efwiel te 


Sm 8 6, 0) (we) © 
Sy <6 6 a) ie), ew) © 
CU OCU Kom 











6.64 4.93 12 42 
4.93 (ORS. 1 ee. 
5.26 Aw24 10.70 
4 60 Pl 8 7228 
6.03 Ar2AT 12.40 
9.10 405)" | L522 
2.41 1.65 4.49 
5-46 2.64 10.21 
9-45 4.84 55233 
7-39 5-39 12.82 
7-03 6.33 15.33 
7.03 [ee 14.19 
Seek Ew 11.99 
9.51 4.76 15.38 
7-27 Ol 12.34 
11.56 4.40 16.51 
52727 Bast 22.2 

6.46 4.90 Lover 
6.41 2 AA LILI 
ri 02 6.40 19.26 
itt 5.11 13491 
fie ee) 22 10.83 
13.44 6.02 21-55 
5-40 A108 tH. 61 
































































































































































98 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
TABLE 8—Maximum and Minimum Soil Thermometers (Bulb at Depth of 
1 Foot below Surface). 
1906. 1907. 
Station I. Station IT. Station I. Station IT. 
Date. Date. 
Min. | Max. | Min. | Max. Min. | Max. | Min. | Max. 
rice a eet re we hy, dak ek re 
Decswio..# 54 56 49.5| 50 Mavis. oo 75 80 5 fe 74 
Deco... 54 56 49 49 May 05.5.5) 73 #89! 69 70 
Dec a2. a 54 57 49 49 May 17.. 75 80 69 73 
Deca. 55 58.5! 49.5] 50 May 2022-2) 775 82 73 a5; 
TABLE 9.—Minimum and Maximum Soil Thermometers (Bulb at Depth of 
1 Inch below Surface). 
Station I. Station IT. Station I. Station II. 
Date. Date. 
Min. | Max. | Min. | Max. Min. | Max. /} Min. | Max. 
1907. sy aie foal abe rg Sey TN oe 1907. OF fod 2 Pic ae Fe. tna Fae 
Marnagoe.: 63 88.5} 60 67 Aprilia. 65 98 62 42 
Mar. 25 22).11N50 go 51 60 May eri: 62 ros 96 62 74 
Mat.5205 52. |ees0 80 45 58 May-s S32. 65 97 64 75 
April 12.6 a7 103 64.5) 71 May 710... 4 G4 97-5| 64 75 
Apri sis! eat 103.54 605 75 
TABLE 10.—Air Temperatures at 1 to 2 Inches above the Surface. 
Station I. Station IT. Station I. Station IT. 
Date. Date. 
Sun. |Shade.) Sun. |Shade. Sun. |Shade.| Sun. |Shade. 
1907. re vise. er, Cre. 1907. oak oF; wie ta ae 
yale e267. 85 65 64 57 April*ro.. 4) 120 98 120 80 
Janae 89 64 70 56 Aprile, . jerae 96 122 85 
Feb, 2 95 65 7% 57 ADVISE oe at rt 88 II4 80 
Heb. Ge. 98 vie 83 65 April oo. >. str 85 103 76 
Rebelo ee 106 76 87 66 AprilieG. 2/1108 80° 1) 114 ai. 
Feb; 13 94 71 78 62 May r.5. 5) Ior a IOI 76 
Feb. 25.. 96 7a 82 64 May? fase. ai a7 90 | 120 80 
Matin Sat a: 93 63 87 52 May »6.454) 98 a, 110 71 
Mar. 9g.. 106 69 86 56 May? © 32.2 J 150 80 rr 768 | 
Mar, I1.. 104 76 96 66 May Io. 114 83 A 78 
Matar se. 153 95, g2 68 May tr37. 97 70 100 68 
Mar. 20 08 105 75 82 65 May 15.. 114 86 124 82 
April’ °3..: 12 90 119 83 May 20.. 116 gI 126 go 









































ENVIRONMENTAL AND HISTORICAL FACTORS. 99 


MEASUREMENTS OF EVAPORATION. 


Recent work by Livingston (1907, 1908) at the Desert Laboratory on 
evaporation and plant development and habitats, in connection with 
observations at stations representing widely different climatic conditions 
in different parts of the United States, indicate the great importance 
of this line of research. 

It is shown by Dr. Livingston in the papers referred to that the evapo- 
rating power of the air depends upon three factors—humidity, tempera- 
ture, and wind velocity—and that measurement of evaporation, which 
is simpler than the measurement of any one of these, gives the directly 
controlling factor in water-loss from plants. For a given soil-moisture 
content, there is a maximum rate of water-supply to any plant through 
root-absorption, and when the evaporating power of the air is great 
enough to cause the rate of loss to surpass that of supply, the plant wilts 
and suffers injury, and death even may ensue. If the rate of transpiration 
approaches or equals that of supply, little or no growth can take place. 

It appears, therefore, that evaporation must be considered as capable 
of affecting the plant directly, as well as through its indirect effect on 
soil-moisture conditions. In arid regions it seems that this effect is par- 
ticularly marked, and that here evaporation actually inhibits the growth 
of many plants, even though the soil-moisture content may be rather 
high. The conclusion necessarily follows that evaporation may exercise 
a determining influence on the distribution of plants in different habitats, 
even in relatively close proximity. 

The methods of experimentation, with full descriptions of the apparatus 
employed, are given by the author in the papers referred to, and it is 
only necessary here to refer briefly to the results, which indicate, as already 
stated, that in any habitat, but especially in arid regions, the evaporating 
power of the air is one of the most important among the conditions which 
determine local distribution, and that the measure of evaporation for a 
given locality also includes some evidence in regard to soil-moisture, which 
is of even greater importance to plant life than the atmospheric conditions. 

Turning for the moment from desert conditions, some comparisons 
of rate of evaporation at certain mountain stations in the neighborhood 
of Tucson and at points in the eastern United States are of great interest. 
The highest instrument on the Santa Catalina Mountains was set up at 
an altitude of 8,000 feet, and showed, for the latter half of May, 1907, a 
weekly rate of evaporation equal to 133 c.c. During the same period 
the instruments at Orono, Maine, and Burlington, Vermont, indicated 
a weekly rate of evaporation of 123 c.c., and 112 c.c., respectively. Such 
correspondence of rate is the more striking in view of the fact stated by 
Livingston (1908, p. 8) that ‘‘the vegetation about the highest instrument 
in the Santa Catalina Mountains possesses the same ecological characters 


100 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


as that in the vicinity of Orono and Burlington, the higher levels of the 
Santa Catalinas being occupied by pine, spruce, maple, alder, elder, cornel, 
brake, columbine, violet, etc. F‘urthermore, plants in Maine and Ver- 
mont were in about the same condition as those at our upper station 
at this time of year.’’ Results of this kind suggest the value and probable 
extensive use of this method of measuring evaporation as a means of 
determining ecological equivalency, whether at near or remote points, 
a matter of fundamental importance, which as yet has received but little 
attention. 

Recent studies of Transeau (1905, 1908) on the relation of plant so- 
cieties to evaporation further emphasize the value of these methods of 
investigation and the importance of rate of evaporation in determining 
the distribution and succession of plants. Earlier papers by the same 
author go to show that a close relation may be traced between the dis- 
tribution of forests in the United States and the combined figures for 
rainfall and evaporation. With no attempt to minimize the edaphic, 
geographic, and historical factors, it is made evident that climatic differ- 
ences which may be expressed in ratios obtained by dividing the mean 
annual rainfall by the depth of evaporation at the same station, correspond 
in a very striking way with the relative position of forest, prairie, and 
arid plains. 


ANIMALS OF THE LABORATORY DOMAIN. 


No comprehensive study of the animals of the Laboratory domain in 
their relations to its plant life has yet been undertaken, though it can not 
be doubted that this would lead to important results. As has been 
shown in the section on means and agencies of distribution, the inter- 
relations of desert plants and animals are particularly close and varied, 
largely advantageous to both, though often quite the reverse. 

Changes of the flora must react, sometimes directly and manifestly, at 
other times indirectly and obscurely, upon the fauna, and vice versa, but 
the history of these relations has never been followed far in this region. 

A few zoologists who have taken up the study of certain groups of ani- 
mals from an ecological standpoint have made valuable contributions. Of 
these may be specially mentioned the recent studies of reptiles and am- 
phibians by Ruthven (1907). His work, too extended to be reported 
here in detail, includes a discriminating account of the habitat relations 
of the various reptiles and amphibians collected in the vicinity of Tucson. 
The habitats described correspond essentially with those defined in the 
present work, and it is significant that notwithstanding the wide range 
of certain species, as for example some of the bull-snakes, which are 
found from the flood-plain of the Santa Cruz River to the pifion zone 
of the mountains, there is a well-established preference, as a rule, for the 
habitat of some special association of plants. Thus the large lizard 
Sceloporus magister is said to be “common on the greasewood plains’’ and 
also to occur, with the ocotillo and sahuaro, at the foot of the Santa Cata- 
lina and Tucson Mountains, though much less common in these latter 
places; so that its principal habitat in this region is preeminently that 
of the creosote-bush association of plants. Similar notes regarding a large 
percentage of other animals taken in the vicinity of both Tucson and 
Alamogordo indicate that many of them at least exhibit a like habitat 
preference. ‘This is expressed by the author in the general statement 
that each set of environmental conditions which is marked out by a 
distinct plant association has a definite reptile fauna. The summary 
with which the paper closes is of much interest, and some of the general 
statements apply equally to both plant and animal forms; it must be 
said, however, that our present knowledge of the distribution of plants 
on the arid plains by no means warrants, for these, the application of some 
ef the theoretical conclusions of the paper cited. The hypothesis “that 
the reptiles of the arid plains have had their origin in this general region 
(Mexican plateau and proplateau), and that the forms of the pifion-cedar 
and pine-spruce associations have been derived from them”’ would give, 
especially as regards the second proposition, a very inadequate, not to 

101 


102 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


say erroneous impression, .1f applied to plant species. Furthermore, it 
can not be doubted that “the arid-plains forms tend to become the climax 
type’’ in the southwestern United States, but in this region, where such 
tendencies are liable at any moment to be corrected by the process of 
volcanic uplift, the future disposition of plant and animal associations 
and habitats can only be tentatively suggested. 

Several recent papers of Professor Wheeler (1907) should also be 
noticed, although they are quite too voluminous to permit more than 
the barest reference. In the course of his investigation of the fungus- 
growing ants of North America, it was found that in the wash near the 
Desert Laboratory a catclaw (Acacia greggit) in the neighborhood of 
the nests of Atta versicolor had been completely defoliated by colonies 
of these ants, which suggests on their part a relation to vegetation which 
may often be prejudicial to the latter, however efficient they may be as 
agents of seed dissemination. Another paper of this writer (Wheeler, 
1907), though not directly discussing questions of plant distribution, 
is of special interest in its bearing on the origin of habits and adapta- 
tions, fundamental as they are in relation to distributional problems, as 
studied in a group of animals which is described as having become the 
dominant invertebrates of tropical America. 

Experiments conducted by Prof. W. L. Tower involving the mutual 
relations of certain plants and animals are now in progress in the vicin- 
ity of the Desert Laboratory, where colonies of Leptinotarsa have been 
established. No report of this can yet be made, but the previous work 
of this investigator, which has already been referred to, is of special 
interest and value as embodying, with much else, a study of a single 
genus of animals with reference to the local distribution and wider migra- 
tions of its species, serving thus to indicate a rational method of pro- 
cedure in following out the geographical history of desert plants. (See 
Tower, 1906.) 

Reference should also be made to the well-known papers of Adams 
(1902) and especially to his views regarding the probable existence of a 
characteristic and varied fauna and flora in the Southwest during the 
Ice Age. These, in general, are in accordance with the results indicated 
in the special contributions presented with this paper and referred to 
in the closing discussion. 


CHAPTER IV. 


VEGETATION GROUPS OF THE DESERT LABORATORY DOMAIN. ' 


The following list includes as nearly-as possible the plants growing on 
Tumamoc Hill, the fenced area of the mesa-like mountain-slopes lying 
to the west, and the Santa Cruz flood-plain between Tumamoc Hill and 
the Santa Cruz River on the east, together with the hydrophytic species 
growing in the Santa Cruz River and adjacent irrigation ditches. For 
convenience the areas noted above have been designated as follows: 
(I) Tumamoc Hill; (11) Mesa-like mountain-slopes; (III) Santa Cruz 
flood-plain; (IV) Santa Cruz River and irrigation ditches. 

The species of each of the above areas have been arranged alphabeti- 
cally under the following vegetation forms: (1) trees; (2) shrubs; (3) 
woody climbers; (4) dwarf shrubs; (5) half-shrubs; (6) perennial herbs; 
(7) biennial herbs; (8) annual herbs including (a) long-lived annuals; 
(b) winter annuals; (c) summer annuals. ‘The occurrence of a species in 
any quantity in an area other than the one of which it is characteristic is 
shown by a Roman numeral indicating the area over which its secondary 
distribution obtains. Besides the four groups of plants corresponding to 
the four areas above noted it is thought desirable to include a fifth com- 
posed of miscellaneous introduced species which have become established 
here by virtue of certain inherent qualities or characters. ‘These exotic 
species are limited almost wholly to area III, though a fewoccur in I and II 

In questions of nomenclature the recommendations of the Vienna 
Congress of 1905 have been followed as closely as possible. For con- 
venience synonyms are given as they stand in the second edition of Heller’s 
Catalogue of North American Plants. 

The writer gratefully acknowledges assistance from numerous sources 
in the preparation of this list of plants. Most of the Graminee have 
been passed upon at one time or another during the last few years by 
Professors Scribner, Hitchcock, and others of the Bureau of Plant Indus- 
try, U. S. Department of Agriculture. Dr. J. N. Rose, of the Smithsonian 
Institution, who during his trip through the Southwest last spring spent 
some time in studying the Cactacez in the field, has determined several 
species which had hitherto remained unnamed. Prof. P. Beveredge 
Kennedy, of the University of Nevada, has kindly verified the species 
of Atriplex; while Mr. Paul C. Standley, of the New Mexico College of 


1Prepared by request and Contributed by J. J. Thornber, A. M., Professor of Botany 
in the Arizona Experiment Station. 





103 


104 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


Agriculture and Mechanic Arts, has worked over the various members 
of the family Allionacea, on which group he is preparing a publication. 
Prof. H. M. Hall, of the University of California, has verified the names 
of miscellaneous plants from time to time from this locality. 


TABLE 11.— Showing the Various Spectes from Standpoints of Habitat and Vegetation Form. 












































igi: II. Tis erie 
| Santa 
| se Santas) (Cruz. Intros 
Vegetation forms. _ Tuma- nome roe Rivér | duced” Total. 
moc Hill. AGE. flood- | and irri-| species. 
plain gation 
| SlOpee: 4 ditches. | 
| ele af 
‘Thees sign be. eae eee | 2 2 UHL , SASS E ae Ate eke 15 
 SHCODS chee 8 eee ee ee | 16 IO LO es sree 3 39 
W Oody twiners-..-) ee RDA per ae Ml Reet eto a? by 3 5 
Jowartshrabs (io). | ra ES SEO es ere BR ey ee, ed 17 
Hali-chrubs< reetage | On al Tek Wa ere I 2 
\iPerennialtherbers., -aakoeoe <i, 2410) ae 7 108 
}) Brenmial hers a. Ses, See | ‘2% I a Wh beer, HAE et eee 3 
Annual herbs: 
(a) Long-lived”... 2.» aes? | 9 250 Whar 2 2 57 
(b) Winter annuals... .| 38 46 TOUeI Se he Be 122 
(c) Summer annuals... 7 25 Ad os ee a 44 
Algae Gat aaa ae le oo I SS Sekar Ih ecgeeeae ene een Po eee et rm 
Total ee eee eed 138 | 128 ri7 T4 52 449 
The data in table 11 suggest some interesting observations. It may 


be taken as almost axiomatic that biennial species are not a success in 
our region. The intense conditions prevailing regularly over a consid- 
erable portion of the year, together with occasional prolonged droughts, 
render their existence almost impossible. Either our plants develop 
root-systems extensive and permanent enough to endure for some years, 
as in the case of many perennial herbs and of all the suffrutescent and 
woody forms; or else their growth is completed within the course ofa 
few weeks, or at most a few months, during those portions of the year 
when mesophytic conditions prevail to some extent over the country. 
There appears to be little middle ground in this matter. In areas I and 
II, where the most pronounced xerophytic conditions obtain, the woody 
species, including trees, shrubs, dwarf shrubs, half shrubs, and woody 
twiners, constitute 30 per cent of the plants. It needs hardly be said 
that to these plant-types belong our characteristic desert fornis. 

The short-lived winter annual and summer annual species, on the 
other hand, make up 43 per cent of the plants of these same two areas. 
That these plants are unable to withstand arid conditions may be in- 
ferred from the fact that with their first approach they cease further 
growth and begin dying. Such species as Phacelia distans, Ellisia torreyi, 
Pectocarya linearis, and Harpagonella palmeri die off in early spring, 
except in the shade of bushes, even with the presence of considerable 


VEGETATION GROUPS OF THE DESERT LABORATORY DOMAIN. 105 


moisture in the soil. As opposed to these, plants like Phacelia crenulata, 
Plagvobothrys pringler, Amsinckia intermedia, and Gulia floccosa are able 
to endure considerable drought by virtue of a better developed root- 
system. They are, nevertheless, species of short duration. As hereto- 
fore noted, growth obtains in these plant-groups during the moister 
portions of the year, after which they disappear entirely from the foot- 
hills and mesa country. Those of the winter annual type, which are 
by far the more numerous, begin to grow in the fall with the advent of 
winter rains and prevailing lower temperatures, such temperature con- 
ditions being essential to their germination,! and continue until well 
in April, after which time they cease to be a factor in the floral covering. 
In contrast with these, the summer annuals begin growth during the 
summer rainy season, when maximum temperatures prevail, and come 
to maturity in the course of six weeks or two months. 

The perennial herbs of these same two areas constitute 22.6 per cent 
of their flora. Since this vegetation form includes 28 per cent of the 
plants of the flood-plain, their presence can not be taken as indicative 
of xerophytic conditions. Many of them are, however, extremely drought- 
resistant. Of the perennial herbs indigenous to areas I, II, and III, 20 
per cent are bulbous, tuberous, or fleshy-rooted species. Among these 
such plantsas A podanthera undulata, Martyniaaltheefolia, Tetraclea coultert, 
Talinum lineare, Rumex hymenosepalus, and Brodiea caprtata begin growth 
earlier and continue later in the season than perennial herbs in general 
without storage organs. Their growth, also, is unchecked with dry spells 
which characterize even our more favorable growing periods. 

The data relating to species growing in the Santa Cruz flood-plain are 
of less value than those of the two former areas, in consequence of agri- 
cultural operations that have been carried on there for many years. The 
distribution, frequence, and abundance of numerous plants have been 
changed; exotic species have found their way in, and not unlikely, 
indigenous plants have suffered eradication. The vegetation forms best 
represented are trees, shrubs, long-lived annuals, and, as already noted, 
perennial herbs. Tor the four areas 11 of the 15 trees occur here and 52 
per cent of the long-lived annual plants for the four areas are indigenous 
to the flood-plain, while 32 per cent more are introduced species growing 
in the same area, making a total of 84 per cent of this type of annual 
plantoccurring here. ‘There can be no doubt that these plants which con- 
tinue their growth during our long season find the deep, alluvial soils of 
_ the flood-plain with their greater water-retaining capacity more conducive 
to their existence than the scant, parched soils of the mesa-like mountain- 
slopes. On the other hand, area III contains 24 per cent of the indig- 
enous shrubs and 6 per cent of the dwarf shrubs and half-shrubs for 








'The writer germinated successfully seeds of winter annual species last summer 
with proper conditions of moisture in a refrigerator; while seeds from the same lots 
with the prevailing summer temperatures remained unchanged. 


106 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


the four areas, as against 68 per cent of the shrubs and g1 per cent of 
the dwarf shrubs and half-shrubs for areas I and II. 

The plants which are characteristic of alkaline situations are ro in 
number, all inhabitants of area III. The most important are members 
of the Chenopodiacee, 3 being shrubs, 2 half-shrubs, and 1 an annual herb. 
Four other families of plants are represented with one species each. 

The exotic plants that have become established in a new country are 
always an interesting group. Especially is this true in a region like our 
own, where the various factors are so sharply defined as to leave little 
doubt concerning the particular character or characters of a plant which 
makes for its success. Of the introduced species taken as a whole, 47 
per cent deport themselves as winter annuals. It is a fact worthy of note 
that with few exceptions these exotic winter annuals are natives of the 
Mediterranean region of the Old World. Our climatic conditions, which 
are quite similar to those of southern Europe and especially northern 
Africa, appear to be so entirely adapted to their manner of growth that 
many of them have little difficulty in securing a foothold here. Several 
of these species, among which are Evodium cicutarium and Hordeum mu- 
rmum, are becoming common plants upon the mesas; especially the former, 
which in many localities in Arizona at this time is more abundant than 
any other species during its period of growth.! Another species worthy 
of mention here is Matthiola bicornis. Seven years ago this plant, appar- 
ently an escape from nearby flower-beds, was represented by a few scat- 
tered individuals on the mesas near the University. At the present time 
it occurs in considerable abundance in this vicinity; besides, it has been 
observed growing on the flood-plain and even across the river on the mesa- 
like mountain slopes. 

The composition of the flora of the Desert Laboratory domain and 
adjacent areas as concerns the more important plant families represented, 
together with the number of species for each, in addition to which there 
are 38 other families with 1 to 3 species each, is as follows: 


Grannnes.. 2 o. 70 Polygonacee....... 12 Plantaginacee..... 5 
Conipositieqs = cece 65 Cichoriates. Geen. II Polypodiaceze...... 5 
Solanaceae Be 16 Nyctaginacez...... 9 Portulacacess © 2a.) 5 
Crititeras 6 eee 16 Ceesalpinacee...... 8 Acanthaces 02) 4 
Euphorbiaceze...... 16 Polemoniacee...... 7 Asclepidacez....... 4 
Boraginacee....... 15 Hydrophyllacee.... 6 Convolvulacee..... 4 
Malvaces..... 0 ee ‘6 PAE ae et 6 Liliacese fa, o2 eee 4 
Chenopodiacez.... . I4 Mimosacee........ 6 Ranunculacee..... 4 
CACtACE iy Werte 13 Onartacée saa... 6 Scrophulariacee.... 4 
Papilionacee....... 12 Amaranthacee..... 5 Zygophyllacee..... 4 





1Thornber, J. J. Alfilaria (Erodium cicutarvum) as a Forage Plant in Arizona. Bull. 
52, Ariz. Exp. Sta., May, 1905. 


VEGETATION GROUPS OF THE DESERT LABORATORY DOMAIN. 107 


The following brief summary will be interesting to the botanist from the 
standpoints of taxonomy, and phytogeography: 


PCE aC EV Ge ES 7 Se ee a ee er ee Oe eee ee ee 68 
paerees ROE TICE ire Mer mre Penn ar ey We oats, 9s oh ss 3, neal Ede Be ea ree ier slesohs 269 
Miduoeto. eeuera Common to both hemispheres 2.0095 5. fem so ws A epeeimns @ as es 126 
Number of genera common to North and South Ameérica........5....2....4.. 58 
Dre ersueSOris yy ester t Senet ei. oe Oe do efece 8 a wel lA Oh Shc g Rie lace dink 39 
Peter wore to uGec OC CUCTO stein. 25.6% cacy seen ecm alive aN overuse sickle ajeus 22 
CONGRATS Ls EL! UES Sa aw, gee calaehelzae a Sie eee ae eta We eat Pe iad © og re 449 
eter ee avec reli SUECICS sc or. sow se aie tie ate Seo On raises alge tee atrans 264 


*Names preceded by an asterisk denote bulbous, tuberous, or fleshy-rooted plants. 
tNames preceded by a dagger denote plants growing in alkaline situations. 
{The Roman numeral at the end of certain lines indicates an additional area on which the species 


thus marked occurs. 
I. Tumamoc HI. 
TREES. 


Cereus giganteus Engelm. 


=Carnegiea gigantea (Engelm.) Britt. 


and Rose. 
Parkinsonia microphylla Torr. 


SHRUBS. 


Acacia constricta Benth. (II).t 
Celtis pallida Torr. (II). 
Echinocactus wislizeni Engelm. (II). 
Fouquieria splendens Engelm. 
Hyptis emoryi Torr. 
=Mesospherum emoryi (Torr.) 
Kuntze. 


Jatropha cardiophylla (Torr.) Muell. Arg. 


Lippia wrightii Gray. 
Lycium berlandieri Dunal. 
Lycium fremontii Gray. 
Opuntia arizonica Griffiths. 
Opuntia blakeana Rose. 
Opuntia discata Griffiths. 
Opuntia leptocaulis DC. 
Opuntia versicolor Engelm. 
Opuntia toumeyi Rose. 
Simmondsia californica Nutt. 


WOODY CLIMBERS. 


Janusia gracilis Gray. 
Nissolia schottii (Torr.) Gray. 


DWARF SHRUBS. 


Aplopappus laricifolia Gray 


= Chrysoma laricifolia (Gray) Greene, 


Ayenia microphylla Gray. 

Cactus grahami (Engelm.) Kuntze. 
Calliandra eriophylla Benth. 
Carlowrightia arizonica Gray. 
-Hermannia pauciflora Wats. 

Hibiscus coulteri Harvey. 

Hibiscus denudatus Benth. (II). 
Krameria canescens Gray. 

Krameria glandulosa Rose. 
Phoradendron californicum Nutt. (II). 
Polygala macradenia Gray. 
Siphonoglossa longiflora (Torr.) Gray. 


I. Tumamoc Hii1-—Continued. 
HALF-SHRUBS. 


Abutilon incanum (Link) Sweet. 
Abutilon lemmoni Wats. 
Arabis eremophila Greene. 
Boerhaavia scandens L. 
Brickellia coulteri Gray 
=Coleosanthus coulteri(Gray) Kuntze. 
Cassia covesii Gray (II). 
Ditaxis sp. 
Dysodia porophylloides Gray. 
Dalea parryi T. & G. 
== Parosela parryi (1. & G.) Heller. 
Encelia farinosa Gray. 
Franseria deltoidea Torr. (II) 
= Geertneria deltoidea (Torr.) Kuntze. 
Galium stellatum Kellogg. 
Haplophyton cimicidium (Pav.) A.DC. 
Hilaria mutica (Buckl.) Benth. (II). 
Menodora scabra Gray (II). 
Parthenium incanum H. B. K. 
Porophyllum gracile Benth. (II). 
Senecio lemmoni Gray. 
Spheralcea pedata Torr. (II). 
Stephanomeria pauciflora (Torr.) (IT) 
=Ptiloria pauciflora (Torr.) Raf. 
Trixis californica Kellogg. 


PERENNIAL HERBS. 


Abutilon crispum (L.) Medic. 
Allionia incarnata L., var. (II) 

=W edelia incarnata (L.) Kuntze, var. 
* Allium reticulatum Don. (11). 
Andropogon contortus L. 
Andropogon torreyanus Steud. (II). 
* Anemone sphenophylla Poepp. 
Aristida divergens Vasey (II). 
Aristida humboldtiana Trin. & Rupr. (11) 
Aristida purpurea Nutt. (II). 
Aristida scheidiana Trin. & Rupr. (II). 
Bouteloua bromoides (H. B. K.) Lag. 
Bouteloua curtipendula (Michx.) Torr. 
*Brodiea capitata Benth. (II). 
*Calochortus kennedyi Porter. 
Cassia bauhinioides Gray (II). 
Cheilanthes wrightii Hook. 


108 


I. Tumamoc Hi__—Continued. 
PERENNIAL HERBS. 


Cheilanthes myriophylla Desv. 

Cottea pappophoroides Kunth. 

*Delphinium scaposum Greene. 

Ditaxis humilis (Engelm. & Gray) Pax. 

Euphorbia capitellata Engelm. 

Euphorbia pediculifera Kngelm. 

Hilaria cenchroides H. B. K., var. 
longifolia Vasey. 

Maximowiczia tripartita Cogn., var. 
tenuisecta Wats. 

Metastelma arizonicum Gray. 

Muhlenbergia microsperma (DC.) Trin. 

Nicotiana trigonophylla Dunal (III). 

Notholena hookeri D. C. Eaton. 

Notholena sinuata (Sw.) Kaulf. 

Pelleza wrightiana Hook. 

Pentstemon wrightii Hook. 

Perezia wrightii Gray. 

*Physalis fendleri Gray. 

*Talinum lineare H. 6B. K. 

Thelypodium sp. 

Triodia mutica (Torr.) Benth. 

Verbena ciliata Benth. 

Vicia hassei Wats. 


BIENNIAL HERBS. 
Aristida sp. (II). 

ANNUAL HERBS. 

Winter Annuals. 


Amsinckia intermedia F. & M. (II). 
Bowlesia lobata R. & P. (II). 
Calycoseris wrightii Gray (II). 
Cryptanthe barbigera (Gray) Greene (II). 
Cryptanthe intermedia (Gray) Greene (II). 
Cryptanthe pterocarya (Gray) Greene. 
Daucus pusillus Michx. (II). 
Ellisia torreyi Gray 
= Eucrypta torreyi(Gray) Heller. 
Erodium texanum Gray (II). 
Eulobus californicus Nutt. 
Evax caulescens Gray, var. (II). 
Filago californica Nutt. (II). 
Galium proliferum Gray. 
Gilia bigelowii Gray 
= Linanthus bigelowii (Gray) Greene. 
Gilia glutinosa (Benth.) Gray. 
Gila inconspicua (Sm.) Dougl., var. sin- 
uata Gray (II). 
Harpagonella palmeri Gray. 
Linum lewisii Pursh. 
Lupinus leptophyllus Benth. 
Malacothrix clevelandi Gray. 
Malacothrix coulteri Gray (II). 
Malacothrix glabrata (D. C. Eaton) Gray. 
Malacothrix sonchoides (Nutt.) T. & G. 
(II). 
Mentzelia aspera L. 


DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


I. Tumamoc Hi~iL—Continued. 
ANNUAL HERBS. - 
Winter Annuals. 


Microseris linearifolia (DC.) Gray (II). 
Nemacladus ramosissimus Nutt. (11). 
Nemophila arizonica Jones. 
Oenothera chamenerioides Gray (II) 
= Spherostigma chamenerioides 
(Gray) Small. 
Parietaria debilis Forst. f. 
Phacelia distans Benth. 
Plantago ignota Morris (II). 
Plantago virginica L,. 
Rafinesquia neo-mexicana Gray (II) 
= Nemoseris neo-mexicana (Gray) 
Greene. 
Silene antirrhina L. 
Spermolepis echinata (Nutt.) Heller (IT). 
Streptanthus arizonicus Wats. 
Thysanocarpus curvipes Hook. 


Summer Annuals. 


Anoda thurberi Gray. 

Boerhaavia intermedia Jones 

Boerhaavia megaptera Stanley. 
Euphorbia florida Engelm. (II). 
Lathyrus pusillus El. 

Leptochloa filiformis (Lam.) Beauv. (II) 


=Leptochloa mucronata (Michx.) 
Kunth. 
Setaria grisebachii Fourn 
=Chetochloa grisebachii (Fourn.) 


Scribn. 


Il. Mgssa-LIkKE MOUNTAIN SLOPES. 


TREES. 


Cercidium torreyanum (Wats.) Sargent. 
Olneya tesota Gray. 


SHRUBS. 


Adelia neo-mexicana (Gray) Kuntze. 
Anisacanthus thurberi (Torr.) Gray. 
Atriplex sp. 
Baccharis emoryi Gray (I). 
Condalia lycioides (Gray) Weberbaur (IIT) 
= Zizyphus lycioides Gray. 
Ephedra trifurca Torr. (1). 
Larrea tridentata (DC.) Coville (I) 
= Covillea tridentata (DC.) Vail. 
Opuntia fulgida Engelm. 
Opuntia spinosior (Engelm.) Toumey. 
Yucca elata Engelm. 


DWARF SHRUBS. 


*Cereus greggii Engelm. 
Coldenia canescens DC. (1). 
Echinocereus fendleri. 
Zinnia grandiflora Nutt. (1) 
= Crassina grandiflora (Nutt.) Kuntze. 


VEGETATION GROUPS OF THE DESERT LABORATORY DOMAIN. 109 


II]. Mkésa-LIKE MOUNTAIN SLOPES— 
Continued. 


HALF-SHRUBS. 


Baccharis wrightii Gray. 
Bebbia juncea (Benth.) Greene. 
Bigelowia hartwegii Gray (I-III) 


= Isocoma hartwegii (Gray) Greene. 


Muhlenbergia porteri Scribner (I-III). 
Panicum saccharatum Buckl. (1). 
Perezia nana Gray. 

Riddellia cooperi Gray 


= Psilostrophe cooperi (Gray) Greene. 


PERENNIAL HERBS. 


Allionia incarnata L,. 

= Wedelia incarnata (L.) Kuntze. 
Aplopappus australis (Greene) 

= Eriocarpum australe Greene. 
*Apodanthera undulata Gray. 
* Aristolochia sp. 
Bahia absinthifolia Benth. (I). 
Baileya multiradiata Harv. & Gray. 
Bouteloua rothrockii Vasey. 
Bouteloua trifida Thurb. (1). 
*Cucurbita digitata Gray. 
Dalea pogonathera Gray 


= Parosela pogonathera (Gray) Vail. 


Euphorbia sp. 
Franseria tenuifolia Gray (IIT) 


= Gertneria tenuifolia (Gray) Kuntze. 


Greenella arizonica Gray. 
Hymenatherum hartwegii Gray. 
*Martynia althezfolia Benth. (1). 
Pappophorum wrighti Wats. 
*Physalis lobata Torr. (III) 
=Quincula lobata (Torr.) Raf. 
Sida diffusa H. B. K. 
Sida hastata A. St. Hil. 


Sporobolus cryptandrus (Torr.) Gray, var. 


flexuosus Thurb. 
*Tetraclea coulteri Gray. 
Tragia ramosa Torr. 
Triodia pulchella H. B. K. (1). 
Euphorbia albomarginata T. & G. (III). 


BIENNIAL HERBS. 
Argemone intermedia Sweet. 
ANNUAL HERBS. 

Long-Lived Annuals. 


Aplopappus gracilis (Nutt.) Gray 


= Eriocarpum gracilis (Nutt.) Greene. 


Aster tanacetifolius H. B. K. (1) 


= Macheranthera tanacetifolia (H.B. 


K.) Nees. 
Atriplex texana Wats. (III) 


= Atriplex tuberculata (Torr.)Coulter. 


Atriplex elegans Dietr. 
Eriogonum abertianum Torr. (1). 
Eriogonum deflexum Torr. 
Eriogonum trichopodum Torr. 
Eriogonum nidularium Coville (?) 
Iva ambrosizfolia Gray (I). 


II. Mksa-LikE Mountrain SLopEs— 
Continued. 


ANNUAL HERBS. 
Winter Annuals. 


Actinolepis lanosa Gray (III). 
Amsinckia tessellata Gray. 
Astragalus nuttallianus DC. (1). 
Beeria gracilis (DC.) Gray (III). 
Calandrinia menziesii (Hook.) T. & G. 
Calyptridium monandrum Nutt. 
Cheenactis carphoclinia Gray. 
Cheenactis stevioides H. & A. 
Chorizanthe brevicornu Torr. (I). 
Chorizanthe rigida (Torr.) T. & G. 
Cryptanthe angustifolia (Torr.) Greene. 
Draba platycarpa T. & G. 
Eremiastrum bellidoides Gray. 
Eremocarya micrantha (Torr.) Greene. 
Eriogonum angulosum Benth. 
Eschscholtzia mexicana Greene (I). 
Evax multicaulis DC. 
Festuca octoflora Walt., subsp. hirtella 
Piper’ (1). 
Gaillardia arizonica Gray. 
Gilia filifolia Nutt., var. diffusa Gray (I). 
Gilia floccosa Gray (I). 
Gilia longiflora (Torr.) Don. 
Hosackia brachycarpa Benth. (I) 
= Lotus humistratus Greene. 
Hosackia humulis (Greene) 
= Lotus humulis Greene 
Lappula redowskii (Hornem) Greene, 
occidentalis (Wats.) Ryd. (1) 
=Lappula occidentalis(Wats.)Greene. 
Lappula texana (Scheele) Greene. 
Lepidium lasiocarpum Nutt. (I). 
Lesquerella gordoni (Gray) Wats. 
Loeflingia pusilla Curran. 
Lupinus concinnus Agardh. 
Malvastrum exile Gray (III). 
Mentzelia albicaulis Doug]. (I). 
Oenothera scapoidea Nutt., var. clave- 
formis (Torr.) Wats. 
=Chylisma scapoidea claveformis 
(Torr.) Small. 
Oenothera czespitosa Nutt. (?) 
= Pachylophus cespitosus 
Raimann. 
Orthocarpus purpurascens Benth., var. 
palmeri Gray. 
Pectocarya linearis (Ruiz & Pav.) DC. (I). 
Pectocarya penicillata (H. & A.) A. DC. (I). 
Phacelia arizonica Gray. 
Phacelia crenulata Torr. (I). 
Plagiobothrys arizonicus Greene. 
Plagiobothrys pringlei Greene. 
Plantago fastigiata Morris (1). 
Salvia columbariz Benth. 
Stephanomeria exigua Nutt. (I and III) 
= Ptiloria exigua (Nutt.) Greene. 
Stylocline micropoides Gray. 
Thelypodium lasiophyllum (H. & A.) 
Greene (1). 


(Nutt). 


110 


II. MkEsa-LIKE MouNTAIN SLOPES— 
Continued. 


ANNUAL HERBS. 
Summer Annuals. 


Amaranthus fimbriatus (Torr.) Wats. 
Aristida americana L. 

Boerhaavia pterocarpa Wats. (III). 
Boerhaavia watsoni Stanley (IIT). 
Boerhaavia thornberi Jones. 
Bouteloua aristidoides (Kunth.) Griseb. 
Bouteloua polystachya (Benth.) Torr. 
Cladothrix languinosa Nutt. (III). 
Cuscuta sp. 

Euphorbia glyptosperma Engelm. 
Euphorbia serpyllifolia Pers. 
Euphorbia serrula Engelm. 
Euphorbia setiloba Engelm. 
Euphorbia sp. 

Euphorbia sp. 

Kallstroemia brachystylis Vail. (III). 
Kallstroemia grandiflora Torr. (IIT). 
Mollugo cerviana (I,.) Seringe. 
Mollugo verticillata L. 

Panicum arizonicum Scribn. & Merrill. 
Panicum hirticaulum Presl. 

Panicum sp. 

Pectis papposa Gray. 

Pectis prostrata Cav. 

Trianthema portulacastrum L,. (III). 


III. Santa CruZ FLOOD-PLAIN. 
TREES. 


Acacia greggii Gray (II). 

Celtis mississippiensis Bosc., var. reticu- 
lata (Torr.) Sargent. 

_ Fraxinus velutina Torr. 

Juglans major (Torr.) Heller. 

Populus fremontii Wats. 

Prosopis odorata Torr. & Frem. 

Prosopis velutina Wooton (11). 

Salix sp. 

Salix nigra Marsh. 

Sambucus mexicana Presl. 

Sapindus drummondi H. & A. 


SHRUBS. 


Atriplex canescens (Pursh) James, 
Atriplex polycarpa (Torr.) Wats. 
Baccharis viscosa (R. & P.) 
= Baccharis glutinosa Pers. 
Cephalanthus occidentalis L. 
Condalia spathulata Gray (I). 
Keoerberlinia spinosa Zucc. 
Lycium andersonii Gray, var. wrightii 
Gray. 
*Lycium fremontii Gray, var. gracilipes 
Gray (?) 
Lycium torreyi Gray (II). 
Pluchea sericea (Nutt.) Coville. 


WOODY CLIMBERS. 


Clematis ligusticifolia Nutt. 
Psedera vitacea (Knerr) Greene. 
Vitis arizonica Engelm. 


DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


III. SAntTA Cruz FLoop-PLaiIn— 
Continued, 


HALF-SHRUBS. 


Acacia filiculoides (Cav.) Trelease (?) 
tSueda moquini (Torr.) Greene 

= Dondia torreyana Wats. 
{Sueeda suffrutescens Wats. 

= Dondia suffrutescens (Wats.) Heller. 


PERENNIAL HERBS. 


Asclepias galioides H. B. K. 
Aster hebecladus DC. 
Aster spinosus Benth. 
= Leucosyris spinosa (Benth.) Greene. 
Boerhaavia viscosa Lag., var. oligadena 
Heimerl. 
*Cyperus esculentus L. 
Chameesaracha coronopus (Dunal) Gray. 
*Datura meteloides DC. 
TDistichlis spicata (L.) Greene. 
Elymus triticoides Buckley. 
Gutierrezia microcephala (DC.) Gray. 
Helenium thurberi Gray. 
*Hoffmanseggia stricta Benth. (II). 
Hymenothrix wislizeni Gray. 
Maurandia antirrhiniflora (Poir.) Willd. 
Panicum obtusum H. B. K. 
Pappophorum apertum Munro (II). 
Philibertella cynanchoides (Gray) Vail. 
Philibertella hartwegii Vail, var. hetero- 
phylla (Engelm.) Vail. 
Physalis longifolia Nutt. 
Ruellia clandestina L. (II). 
*Rumex berlandieri Meisner. 
*Rumex hymenosepalus Torr. (II). 
Sida lepidota Gray, var. sagittefolia Gray. 
Solanum douglasii Dunal. 
Solanum eleagnifolium Cav. (II). 
Solidago canadensis L., var. arizonica 
Gray. 
Spheralcea cuspidata Gray (Britton) (II). 
Sporobolus wrightii Monro. 
Tencrium canadense L. var., angustatum 
Gray. 
Teucrium cubense L,. 
Trichloris fasciculata Fourn. (II). 
Verbena canescens H. B. K. 
Setaria composita H. B. K. (II) 
= Cheetochloa composita (H. B. K.) 
Scribn. 


BIENNIAL HERBS. 
Mentzelia wrightii Gray (II). 
ANNUAL HERBS. 
Long-Lived Annuals. 


Ambrosia aptera DC. 
Anoda cristata (L,) Schlecht. 
= Anoda lavaterioides Medic. 
Aster exilis Ell. 
Aster incanus (Lindl.) Gray 
=Macheranthera incana (Lindl.) 
Gray. 


VEGETATION GROUPS OF THE DESERT LABORATORY DOMAIN. 


Ill. Santa Cruz FLoop-PLsAIn— 
Continued. 


ANNUAL HERBS. 
Long-Lived Annuals, 


yAster parviflorus Gray 
= Macheranthera parviflora Gray. 
Atriplex bracteosa Wats. (?) 
ytAtriplex sp. 
Chenopodium leptophyllum Nutt. 
Chenopodium fremontii Wats. 
Conyza coulteri Gray (II). 
Cuscuta salina Engelm. 
Cyperus ferax Rich. 
Eclipta alba (L.) Haussk. 
Erigeron divergens T. & G. 
Euphorbia preslii Guss. 
Helianthus annuus L. 
Helianthus petiolaris Nutt. 
Heterotheca subaxillaris (Lam.) Britton 
& Rusby. 
Leptochloa imbricata Thurb. 
Lepidium thurberi Wooton. 
Martynia sp. 
Nama hispidus Gray 
= Conanthus hispidus (Gray) Heller. 
Parthenice mollis Gray. 
Petunia parviflora Juss. 
Portulaca retusa Engelm. (II). 
Samolus floribundus H. B. K. 
Verbesina encelioides (Cav.) B. & H. 
tWislizenia refracta Engelm. 


Winter Annuals. 


Androsace occidentalis Pursh (?) 
Bromus carinatus H. & A., var. arizonicus 
Shear. 
Corydalis aurea Willd., var. occidentalis 
Engelm. 
=Capnoides montanum (Engelm.) 
Britton. 
Hordeum pusillum Nutt. 
Lepidium sp. 
Monolepis nuttalliana (R. & S$.) Wats. 
Myosurus minimus L. 
+Oligomeris glaucescens Camb. (II). 
Phalaris caroliniana Walt. 
Platystemon californicus Benth. 
Poa bigelowii Vasey & Scribn. 
Polypogon monspeliensis (L.) Desf. 
Sisymbrium canescens Nutt. (I and IT) 
= Sophia pinnata (Walt.) Howell. 
Sisymbrium incisum Engelm. 
= Sophia incisa (Engelm.) Greene. 
Spheralcea coulteri (Wats.) Gray. 
Veronica peregrina L. 


Summer Annuals. 


Amaranthus palmeri Wats. 
Chloris elegans H. B. K. (II). 
Cyperus aristatus Rottb. 

= Cyperus inflexus Muhl. 
Eragrostis neo-mexicana Vasey (II). 


111 


III. Santa Cruz FLoop-PLAIn— 
Continued. 


ANNUAL HERBS. 
Summer Annuals, 


Eragrostis pilosa (I..) Beauv. (II). 
Eriochloa punctata (I,.) W. Hamilt. 
Ipomoea coccinea L,. 
= Quamoclit coccinea L. 
Ipomoea hederacea Jacq. 
Leptochloa viscida (Scribn.) Beal 
Leptochloa filiformis (lam.) Beauv., var. 
=Leptochloa mucronata (Michx.) 
Kunth. 
Panicum fuscum Sw. 
Physalis angulata L., var. linkiana (Nees.) 
Gray. 


IV. Santa Cruz RIVER AND IRRIGATION 
DITCHES. 


PERENNIAL HERBS. 


Agrostis verticillata Vill. 
Hydrocotyle ranunculoides L. f. 
Oenothera rosea Ait. 
= Hartmannia rosea (Ait.) Don. 
Paspalum distichum L,. 
Potamogeton pusillus L. 
Radicula nasturtium-aquaticum (L.) Brit- 
ton & Rusby 
= Roripa nasturtium (L.) Rusby. 
Zannichellia palustris L. 


ALGAY, 


Cladophora sp. 
Closterium sp. 
Hydrodictyon sp. 
Oedogonium sp. 
Penium sp. 
Spirogyra sp. 
Vaucheria sp. 


MISCELLANEOUS INTRODUCED SPECIES. 


SHRUBS. 


Arundo donax L. 
Nicotiana glauca Graham. 
Poinciana pulcherrima Sw., var. flava. 


HALF-SHRUBS. 
Marrubium vulgare L. 
PERENNIAL HERBS. 


Convolvulus arvensis L. 

Cynodon dactylon (L.) Pers. 
=Capriola dactylon (L.) Kuntze. 

Malva parviflora L. 

Plantago major L. 

*Rumex crispus L. 

Sorghum halepense (L.) Pers. 


112 


III. Santa Cruz FLoop-PLAIn-— 
Continued. 


ANNUAL HERBS. 
Long-Lived Annuals. 


Amaranthus blitoides Wats. 
Amaranthus grecizans L,. 
Chenopodium album L,. 
Chenopodium murale L. 
Digitaria sanguinalis (L,.) Scop. 
= Syntherisma sanguinalis (L.) Dulac. 
Echinochloa colona (L.) Link. 
Echinochloa colona (L.) (Link), 
zonale (Guss.). 
Echinochloa crus-galli (L.) Beauv. 
Eragrostis megastachya (Keeler) Link. 
= Eragrostis major Host. 
Erigeron canadensis L. 
=Leptilon canadense (L.) Britton. 
Gaura parviflora Dougl. 
Ipomeea purpurea (L.) Roth. 
Panicum capillare L. 
Polygonum aviculare L., var. 
(Link) Koch. 
= Polygonum littorale Link. 
Polygonum lapathifolium L. 
Portulaca oleracea L. 
Sonchus asper (L.) All. 
Sonchus oleraceus L,. 
Tribulus terrestris L. 
Xanthium commune Britton. 


var. 


littorale 


DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


III. Santa Cruz FLoop-PLAIn— 
Continued. 


ANNUAL HERBS. 
Winter Annuals. 


Anthemis cotula L. 
Avena fatua L,. 
Brassica nigra (1,.) Koch. 
Bromus maximus Desf., var. Gussoni Parl. 
Bromus rubens L. 
Bromus unioloides (Willd.) H. B. K. 
Capsella bursa-pastoris (L,.) Medic. 
=Bursa bursa-pastoris (L.) Britton. 
Centaurea melitensis LL. 
Erodium cicutarium (L.) L’Hér. 
Festuca myuros L. 
Gilia chamissonis Greene. 
Hemizonia fitchii Gray 
= Centromadia fitchii (Gray) Greene, 
Hemizonia wrightii Gray 
= Deinandra wrightii (Gray) Greene. 
Hordeum murinum L,. 
Lamarckia aurea (I,.) Moench. 
Lolium temulentum L. 
Matthiola bicornis (Sibth.) DC. 
Matricaria suaveolens (Pursh) Buchenau 
= Matricaria matricarioides (Less.) 
Porter. 
Medicago hispida Geertn. 
= Medicago denticulata Willd. 
Melilotus indica (L.) All. 
Poa annua LL. 
Silene gallica L. 
=Silene anglica L. 


CHAPTER V. 


THE ORIGIN OF DESERT FLORAS.' 


The climatic and physiographic features which characterize deserts 
comprise combinations of meteoric and orographic factors to produce a 
rainfall markedly less than the possible evaporation, low relative humid- 
ity, comparatively small vertical increase in soil-moisture, low humus 
content of the soil, undeveloped surface drainage resulting in restricted 
areas highly charged with salts, comparatively great diurnal variations 
in both soil and air temperatures, and marked effects of wind movement 
and erosion on the surface. Many of the plants and animals of such 
regions exhibit distinctive habits and specialized structures articulating 
with the conditions of the limiting environmental factors enumerated. 

It is not to be taken for granted, however, that all of the organisms 
native or resident within a region properly designated as a desert show 
marked xerophilous qualities. On the contrary, it is quite possible 
that species suitable by form and structure for existence under conditions 
furnished by regions of great precipitation may find wide distribution in 
arid areas. This occurs, however, only when the scanty precipitation 
comes within a limited part of the year, giving a rainy season, or period 
of maximum precipitation, in which such forms may carry out their 
entire cycle of activity and then pass into dormancy, remaining qutes- 
cent in the form of seeds or modified shoots during the intervening drier 
seasons. It is to be noted, however, that such forms have peculiar 
rhythms and respond to changes in moisture and temperature in a man- 
ner not common to mesophytic species. 

A number of forms are to be found in every desert, having structural 
features which give them a distinct aspect, and it is to these that refer- 
ence is usually made in the characterization of the flora of any arid region. 
Such plants have many physiological capacities definitely suitable for 
activity during the drier seasons, and may in fact remain inactive in 
the periods furnishing conditions suitable for the activity of mesophilous 
forms. It is the origination of the qualities and characters which dis- 
tinguish these species that invites attention in the present connection. 

The first step in such an inquiry would naturally consist in going back 
over the paleobotanical records in the search for fossils of species, which 
might show evidences of having lived in an arid climate in previous 
epochs. Existing information as to the contour and profiles of ancient 
land masses gives every justification for the belief that extended areas 








1 Prepared by request and contributed by Dr. D. T. MacDougal, Director of Botani- 
cal Research, Carnegie Institution of Washington. 
113 


114 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


must have been subject to periods of comparative aridity. Attempts 
to recognize xerophilous characters in the fossil plant remains have met 
with but little success, however. Some of the plants of the Carbonifer- 
ous period had fleshy bodies, with small, central woody cylinders, great 
development of pulpy ground-tissue and scaly appendages, suggestive 
of the reduced foliar organs of desert plants, but currently accepted 
interpretations place these ancient Lycopods and Calamites as residents 
of regions with abundant supplies of moisture, or perhaps as actually 
living in swampy areas. 

There seems to be no escape from the conclusion, therefore, that if 
desert conditions did occur in previous epochs, the only types of vege- 
tation found in them must have been those which were active during 
the seasons of maximum precipitation as described above. The con- 
sideration of a still wider range of facts leads to the inevitable conclusion 
that the form-characters, moisture conserving capacities and resistance 
to desiccation distinctive of xerophytic species must have made their 
appearance within comparatively recent geologic time. 

Our investigations of this matter may therefore be restricted to a 
comparatively modern phase of the history of the earth. ‘The desert 
conditions to be encountered on the earth’s surface at the present time 
are not to be taken as having come about by a simple, direct, and con- 
tinuously increasing aridity. An accumulation of evidence from the great 
central basin of Asia, from southern Australia, northern and southern 
Africa, and from the basins and plains of western America tends to estab- 
lish the conclusion that the climate has undergone oscillations between 
periods of lower temperature and maximum precipitation and of maxi- 
mum aridity with increased variations in temperature, the swing from 
one maximum to another occupying periods of a thousand, two thousand, 
or many thousands of years. 

Furthermore, the main movement in the change of climatal condi- 
tions may be complicated by minor fluctuations of such pronounced 
character as to obscure the total movement for hundreds of years. Thus 
the climate of the Great Basin in western America is supposed to be 
undergoing a change toward increased humidity, but it can not be said 
whether this may be part of a major change which will finally carry the 
climate to a condition of great precipitation and equable temperatures, 
or whether it may be simply a minor variation of the change toward 
a period of still more marked aridity. 

The extremes reached in such variations are so wide apart as to pred- 
icate an almost total change in the character of the floras of the regions 
concerned, and it may be safely assumed that but few Species now inhabit 
the great basin of Nevada which grew along the shores or in the tribu- 
tary valleys of Lake Lahontain. A large proportion of the flora doubtless 
originated during the present arid cycle, although of course some dis- 


ORIGIN OF DESERT FLORAS. 115 


semination must have occurred from surrounding regions. Neither the 
survivors nor the invaders, however, would include the peculiarly xero- 
phytic forms by which any desert is characterized. 

The problems involved in the study of the origination of these forms 
comprise all of the main questions as to the methods, procedure, and 
actuating causes in evolutionary movements, and are by no means to be 
solved within the brief limits of this paper. The known effects of some 
of the factors concerned and the major limitations of the whole subject 
may be profitably outlined, however. 

An adjustment of perspective may well be made before going farther 
into the qualities of desert vegetation. Most of the contact of the human 
race with plants, leading into a consideration of their intimate and gen- 
eral nature, has taken place in the climates of moist temperate and 
tropical zones, where the features encountered are unconsciously assumed 
to be normal, typical and average, while those presented by other regions, 
including alpine zones and desert areas, are regarded as adaptational, 
highly specialized and abnormal. ‘The coating of stems and leaves with 
wax, hairs or spines, is no more highly adaptational, however, than the 
formation of bark on trees, or leaf excision in deciduous plants, and 
scores of other features offered by species of the moist temperate regions, 
and in most respects they do not depart more widely from primitive 
morphological types. If the structures offered by plants of a moist 
forest of the Mississippi Valley had been first dealt with by the trained 
scientists of a desert-dwelling people, much would have been made of 
their non-typical and adaptive character. The fact is not to be lost 
sight of, however, that the general progress of conditions has been toward 
desiccation and not increased precipitation, and that in a general way 
the xerophyte is one of the recent developments of the vegetable kingdom. 

The relation existing between the structure, general form, and habits 
of an organism and the environmental conditions encountered by it are 
extremely difficult to decipher. In no branch of biological science are 
plausible explanations so easily framed, and in no interpretations of 
natural phenomena is it so easy to go astray. It seems to be tacitly 
assumed by the majority of writers that when a plant is taken into a 
new environment it undergoes changes which fit it better for that envi- 
ronment. This does not always follow. Thus, if a mesophyte is com- 
pelled to develop under arid conditions, the shoot formed will differ more 
or less markedly from that which might have been formed under the 
average conditions to which the species was accustomed. ‘This departure 
is in no sense an adaptation, but is the direct result of the balance 
established by the absorbing organs, the conducting stems, and the 
transpiratory surfaces with regard to the absorption, formation, trans- 
portation and transpiration of water and other food-material. Restricted 
surfaces of the shoots might be an advantage and offer increased suit- 


116 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


ability for the new conditions, but this advantage is offset by the fact 
that other members, such as the root, have also undergone a_restriction 
with the general result of dwarfing and no increase in efficiency in dealing 
with the newly encountered conditions. 

Over 100 species of seed-plants were grown in complete darkness and 
under faint illuminations in the experiments upon etiolation completed 
three years ago. The action of light was seen to act as a stimulus set- 
ting in action morphogenic processes of unusual character, by which the 
stems of some species were enormously thickened, in others unduly 
elongated, while nearly all showed various useless atrophies and hyper- 
trophies in which the supply of reserve food-material was used to no 
perceptible advantage. Illustrations of a similar character might be 
multiplied indefinitely. 

Not only are the direct ontogenetic and morphogenetic responses of 
plants to environmental forces not necessarily of an adaptive character, 
but it is impossible to connect some of the most highly specialized and 
heritable structures with the supposedly causal conditions which they 
meet. Spines and glochids do lessen the ravages of grazing animals 
upon cacti to some extent, but such structures seem to be induced by 
aridity or poverty of available water, and the deterioration of leaves 
and branches has been carried farther in a dozen native species, with 
the result that we have that many nearly unarmed forms in American 
deserts. This water-relation alone may not be taken to account for the 
form and habits of the cacti, since many members of the family inhabit 

tropical forests and regions of pronounced rainfall. 

’ In the case of the cacti, the forms and capacities which have been 
taken on differ so completely from those of any possible ancestor to 
which they might be traced, that it is extremely difficult to place them 
in a phylogenetic system. The appearance of various members of this 
family in widely separated regions and under such greatly different cli- 
matic conditions suggests a derivation from some group of morphological 
types having within them the possibility of vegetative development with 
reduction of the branching of the shoot and the increase of the body 
resulting in succulency and capacity for water-storage. The appearance 
of such notable differentiation within such a comparatively brief time 
is perhaps to be taken as one of the most rapid and notable evolutionary 
developments which has yet been brought up for consideration. 

It has been shown above that the changes undergone by the shoot of 
a plant when it is brought into an unusual environment may or may not 
lead to increased efficiency in meeting the conditions which induce them, 
the entire matter resting upon the fact that when a stimulus consisting 
of a change in intensity of temperature, light, soil-solutions, or humidity, 
is brought to bear upon a plant it responds as a living machine, while 
at the same time the direct effect of the external force upon the parts of 


ORIGIN OF DESERT FLORAS. Laz 


the machine or its processes may materially modify their properties, or 
cause radical alterations in the processes themselves. 

These adjustments of function and alterations in structure are no more 
than the response of the individual, however, and are not transmitted 
to its descendants unchanged. The transfer of a plant from a warm to 
a colder climate, or from a moist to an arid region, may result in altera- 
tions which leave some effect when the plant is returned to its original 
habitat, lasting perhaps for two or three generations, but so far proof is 
lacking that any irreversible modification has been caused by the action 
of external conditions upon the soma or vegetative part of a plant. It 
would therefore be unsafe to assume that the desert flora owes the origin 
of any of its constituents to gradual adaptations resulting from the 
action of these conditions. 

Another aspect of the subject remains to be considered. In addition 
to the direct physiological, somatic response of a plant expressed in its 
roots, stems, or leaves, it is to be borne in mind that many of the agen- 
cies which cause these variations also act directly upon the germ-plasm, 
which gives a direct response, and not of an adaptive nature. 

By the controlled application of climatic factors, Tower has been able 
to induce the appearance of strains of beetles which diverge from the 
parental type in one main character, with correlated variability in others, 
which crossed readily with the parent and might be swamped by it. 
By the application of reagents to the reproductive elements of seed- 
plants I have been able to cause the appearance of new forms of plants 
which diverge from the parent in several qualities, which do not readily 
cross with the parent, and the newly acquired characters are irreversible 
so far as the tests of three generations may be taken as conclusive. Re- 
sults of a similar nature have been obtained by Gager with the use of 
radium preparations. The direct effects might consist in a modification 
of the relative speed or precession of the various processes in the gamete, 
or of the activities of the chain of enzymes, or of some disturbances of 
the delicate balance among the ions in the protoplasm. Still beyond 
these, stimulative responses or reactions may occur, which would find 
their ultimate expression in deviations in external form of the progeny 
of an individual subjected to the unusual agencies in question. Changes 
of the same nature may well be induced by many conditions to which 
plants are naturally subject, the survival of the resultant forms being 
solely a matter of possible interbreeding and habital selection. 

- A stream takes its rise near the alpine plantation of the Desert Labo- 
ratory and flows out on the desert a few miles away and a mile lower 
down. Doubtless hundreds and thousands of seeds are carried to the 
lowlands each year. Some of these develop into individuals which carry 
out reproduction. This is usually done in the native habitat, at actual 
temperatures of the tissues not above 60° or 70° F. Down below, spore 


118 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


formation, reduction divisions, and fertilization may ensue in tempera- 
tures 40° or 50° higher, a difference capable of being endured by the 
shoots of some plants now being tested, and which might well cause irre- 
versible developmental changes. Other factors of the environment may 
operate in a similar manner. 

It is therefore to be seen that all of the available evidence of a direct 
character tends to show that the alterations of shoots and vegetative 
members in general in response to the stimulation of external factors 
may or may not be of such nature as to result in increased fitness to the 
conditions concerned. Also, that such changes are not fully transmis- 
sible to succeeding generations and are in no sense irreversible. Adap- 
tation, therefore, furnishes but an insecure basis upon which to found 
a theory of the origin and development of any flora, inclusive of those 
characterizing arid regions. 

The influence of external conditions upon the germ plasm, however, 
has been seen to produce irreversible changes in a hereditary line by 
which new combinations of qualities and new characters were called 
out, which were fully transmissible. Furthermore, the newly produced 
forms perished in some localities endured by the parental type, but 
exceeded it in weathering the conditions of other localities. If to these 
facts it is added that one of such induced types which has been studied 
most thoroughly does not readily hybridize with the parent, even when 
growing with the branches interlocked, it is to be seen that the possi- 
bilities of origination and survival of new forms in any locality are very 
great. 

‘ Newly induced forms thrown into the complex conditions of an envi- 
ronment or locality do not necessarily survive because of qualities which 
fit them for competing with and crowding out other forms, but rather 
by reason of capacities suitable for the physical factors encountered. 
By reason of this they may, in reality, simply occupy a vacant niche 
in the system of living things present. The features of the competition 
ensuing when a new form is projected into the dense associations of 
mesophytic zones are lacking to some extent in deserts, since not so 
much crowding ensues as to the utilization of light and food-material. 
The chief struggle is not primarily among the various species of xeroph- 
ilous plants, but is waged between plants and animals over the water- 
supply. During the seasons of greater precipitation animals find food 
and water in plenty with but little disturbance to the plants which are 
essentially desert forms. With the coming of the arid seasons accom- 
panied by the death of the forms which furnish much of the food and 
the diminution of the water-supply, the animals are driven to make use 
of every possible source of water and food. One resource is to be found 
in the tender, turgid seedlings of the succulents which are eaten by the 
millions in their unprotected condition. If it were possible to exclude 


ORIGIN OF DESERT FLORAS. 119 


mammals and birds from the domain of the Desert Laboratory for a few 
years, it would undoubtedly include dense forests of sahuaro. 

Xerophilous types of vegetation are of comparatively recent origin, 
yet at the present time suitable conditions for them are furnished by 
deserts, the total area of which is equal to that of a large continent. The 
further desiccation of the earth’s surface, premised by many geologists, 
will give extended conditions for the origination and survival of addi- 
tional forms, so that the movement toward xerophily may be reckoned 
as one of the most important in evolutionary procedure at the present 
time. 


‘ rf 


hits 4 ces #6 es ee 3 


i 


me Saale? 





CHAPTER VI. 


REVIEW AND DISCUSSION. 


In conducting the investigations which are discussed in the present 
paper the work was carried out as follows: first, a survey of the Labora- 
tory domain was made, in order to recognize by actual observation its 
natural associations of plants, and to gather up, in the most general 
way, through such observation, whatever evidence could be obtained 
regarding the factors by which these associations have been determined; 
second, a more thorough study of certain constituent species of the asso- 
ciations was undertaken, in which detailed mapping, records of local 
distribution, and investigation of the relation of certain physical factors 
to choice of habitat on the Laboratory domain were supplemented by 
comparative observations at various points in New Mexico, Arizona, and 
southern California; finally, the aid of specialists was called in for the 
purpose of gaining more exact knowledge of the soils, of geological history, 
and of the plants themselves, in their biological and genetic relations, so 
far as these might be expected to throw light on the problem in hand. 

Less reference, possibly, than might have been expected has been made 
thus far in regard to the general subject of plant migration. This, 
however, has not been neglected; and the more important results, so far as 
they pertain to the immediate subject, have occasionally been cited. 
Further reference to these, suggested in part by the analysis of the flora, 
will be made in the following pages. 

We are nowin a position tocall attention to facts that may be regarded 
as satisfactorily established, and to point out their more obvious relations 
to distributional problems. 


THE PLANT ASSOCIATIONS. 


It has been seen that the plant associations and habitats of Tumamoc 
Hill and the adjacent valley fall naturally into four well-defined groups, 
namely, those of the river, of the flood-plain, of the slopes, and of the hill. 

Associations of the rvver.—These are two in number: first, the aquatic 
plants of the river itself and the irrigating ditches fed from it, and second, 
the species of the river-banks. These latter include not only willows 
and cottonwoods, but arrow-weed and some other species characteristic 
of the arid Southwest. The small number of species belonging to both 
of these associations, in comparison with those of corresponding habitats 
in regions of greater rainfall, is significant. It is apparently due to two 
causes; first, the greater difficulty which seedlings encounter in starting 

121 


eae DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


where the water-supply is intermittent, and second, the high rate of 
evaporation in connection with the inconstant water-supplv. . 

Associations of the flood-plain.—No associations are more characteristic 
than the two which belong to the flood-plain of the Santa Cruz and 
other rivers of the same region, nor could there well be a case in which 
the delimitation of the smaller area occupied by one of these associa- 
tions from the larger one in which it lies is more clearly referable to 
special conditions. In this case accumulations of alkali salts, commonly 
in areas of defective drainage, constitute the essential peculiarity of the 
habitat of the salt-bushes. 

Outside of these areas the flood-plain is the habitat of an association 
composed of two groups of plants of very different biological require- 
ments. These are, on the one hand, the mesquite and Acacias, whose 
long roots extend to depths where a sufficient water-supply is assured, 
and on the other, the Bzgelowia and various other plants of low growth, 
the roots of which occupy more superficial soil-layers and are, as far as 
soil relations are concerned, subjected to more distinctively xerophytic 
conditions. These differences of ecological requirements in plants closely 
associated on the same ground are of fundamental importance as regards 
competition. 

Associations of the slopes——To this group belong the creosote-bush 
association, the palo verde-catclaw association of the wash, and the asso- 
ciation of F’ranserva and cholla. The different behavior of certain species 
of these associations as regards strict maintenance of habitat choice is 
especially worthy of note. While the catclaw, for example, hardly passes 
- beyond the limits of the wash, the creosote-bush not only invades this, 
but grows there far more luxuriantly than within its own special habitat. 
The relations are complicated, but it is plain that this behavior of the 
creosote-bush is the result of a greater capacity of adjustment on its part 
to differences in amount of soil-water than is possessed by such plants 
as the catclaw and palo verde. 

Associations of the hill.—Up to this point it is manifest, even upon 
casual observation, that soil relations have had a preponderating influ- 
ence in determining the different associations of plants and their limits. 
On coming to the hill, however, it is apparent that another factor, namely, 
aspect, or direction of slope, has also exerted a marked influence in deter- 
mining the composition and place of several of the associations. ‘This 
is seen with special clearness in the case of the Lippia association and 
that of Cereus giganteus and Encelia farinosa, the former occurring here 
exclusively on north exposures, the latter well represented only on the 
east, west, and especially on the south sides of the hill. 

The association of annuals, belonging to superficial soil-layers, though 
well represented on the hill, is not confined to it. Temperature and 
soil-water are obviously the factors which determine the appearance in 


REVIEW AND DISCUSSION. 123 


their seasons of the plants composing the two widely different biological 
groups of this association. 

To avoid unnecessary diversion the parasitic, symbiotic, and miscel- 
laneous introduced species are here omitted from discussion. Excluding 
these, there are in the four natural groups that have been described 12 
well-defined associations of plants exhibiting habitat preferences which, 
on the evidence obtained from careful observation, must be correlated 
first of all with soil-water, but in some cases quite as clearly with tem- 
perature. Other factors, especially aeration and percentage of alkali salts, 
apparently have a dominant influence in one or more of these associations. 

As already noted, it has been observed that in several of the associa- 
tions plants of widely different physiological requirements grow in close 
proximity. Some of the more conspicuous cases are the Brgelowra, which 
on the flood-plain covers the ground under the mesquite, and Parkin- 
sonia microphylla, which is one of the most constant associates of the 
sahuaro on the hill. It has also been observed that some species, notably 
the creosote-bush and one or more of the salt-bushes, are by no means 
confined to their own special habitats, but grow even more vigorously 
in quite different situations beyond their limits. These phenomena are 
significant and are referred to in a later section. 


EDAPHIC RELATIONS. 


The conclusions arrived at in the study of the plant associations of 
Tumamoc Hill and the adjacent valley, though based on long-continued 
observation, nevertheless required confirmation. The work, therefore, 
was extended so as to include detailed mapping and more thorough 
study of the local distribution of certain species, comparative observa- 
tions in the Gila Valley and elsewhere, temperature determinations, and 
finally a special investigation of the soils of the Laboratory domain. 

Examination of the maps (plates 13-18) in connection with the text, 
shows that, in the local distribution of the species selected, character of 
soil and, in certain cases, aspect preference, are evidently two most impor- 
tant factors. The preference of Encelia farinosa and Cereus giganteus 
for southern exposures is strikingly shown by the maps (plates 13, 15, 
and 16). Equally clear is the fact, shown by plates 14 and 18, that 
the creosote-bush and mesquite exhibit no aspect preference whatever. 
On the other hand, the sharp delimitation of the areas of thickest growth 
of these two species, in connection with the great differences of soil on 
the flood-plain and the Tumamoc slope, renders it hardly possible to 
avoid the conclusion that soil relations have determined their respective 
habitats. Through all the careful work which was done in connection 
with the mapping of distribution of species, these two facts, the deter- 
mining influence of soil conditions on the one hand and of aspect on the 
other, have been constantly emphasized. 


124 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


Comparative studies, especially those carried out in the upper Gila 
Valley in connection with the Bureau of Soils, U. S. Department of 
Agriculture, show the existence there of the same topographic features, 
though on a larger scale, and the same general grouping of plant asso- 
ciations as in the Santa Cruz Valley. There is, moreover, a striking 
correspondence between the location of the various plant associations 
and that of the independently mapped areas marking soil distribution 
and character. An extended study of these areas, supplemented by a 
similar study in the Salt River Valley, has led to the conclusion that 
water-content, percentage of alkali, and drainage, that is to say aeration, 
are the three soil factors that have most obviously influenced the local 
distribution of desert plants and governed their association in definite 
habitats. 

The soil investigations conducted on the Laboratory domain by Dr. 
Livingston have thus far been directed chiefly to a determination of the 
water-content, but in this respect they confirm to a noteworthy degree 
the conclusions based on comparative observation. 

Four distinct types of soils are distinguished, namely, those of the hill, 
of the Larrea slope, of the wash, and of the flood-plain, corresponding 
closely with the grouping of plant associations already described. It is 
shown that the soil of the hill absorbs relatively most water and holds 
it with great tenacity. It dries only by the action of plants and by 
direct evaporation at its upper surface. The soil of the flood-plain holds 
as much water at the start, but dries downward as well as upward; that 
is, the dry soil below absorbs water from the surface layers, so that it is 
.not to be expected that this soil will remain wet, at a foot depth say, 
as long as the soil of the hill. The soil of the Larrea slope is much like 
that of the hill, but has so much angular gravel mixed with it that its 
absorbing and holding capacity are alike low. It has excellent drainage 
conditions, and even if once thoroughly saturated does not remain so 
for a long period. ‘The wash, as indicated by determinations of water- 
content, is even worse off than the Larrea slope, but its perennial plants 
apparently root deeply enough to get at the underground flow. 

Within these several areas modifications of conditions are pointed out 
which correspond closely with observed facts of habit and distribution. 
Thus the underground flow of the wash and the flood-plain is available 
to such deeply rooting species as the acacias and the mesquite, and 
these areas accordingly are their special habitat. The fact that on these 
areas the soil near the surface dries out relatively rapidly corresponds 
with their occupation by two biological groups of plants very different 
from the deeply rooted perennials and different from each other, namely, 
low perennials of distinctly xerophytic character, and various annuals, 
which are commonly able to complete their short cycle of development 
before conditions of extreme drought set in. Neither of these plant 


REVIEW AND DISCUSSION. 125 


groups is confined to the flood-plain and wash, but their habits and 
physiological requirements are such as to make these habitats available 
for their occupation. 

The account that is given of the march of the soil-moisture content 
is no less important in its bearing on the relations of desert plants to 
soil-moisture. The lagging of the effect of precipitation behind the 
march of the precipitation itself, and a lagging in the opposite direction, 
graphically shown by curves, go far to explain the fresh condition which 
plants of this region, particularly those of the hill, exhibit after a com- 
paratively long period in which no rain has fallen. 

An important observation to the effect that the moisture conditions 
of the surface layers of the soil must be regarded as the prime factor in 
determining germination, thereby imposing inevitable limitations to what 
can be accomplished by the plants of the Laboratory domain in even 
making a start toward its continued occupation by a natural renewal, 
corresponds perfectly with what is observed, for example, in the deport- 
ment of the winter and summer annuals, which has already been described. 

Without entering further into details, which, however, can be given 
in any number by one familiar with the ground, it may be said that so 
far as relates to determinations of soil-moisture the facts established by 
this investigation show a remarkable degree of correspondence with the 
facts of distribution as already observed. Such correspondence can not be 
accidental, and the only possible explanation points to a definite causal 
relation. 

It is to be regretted that it was not found practicable to extend the 
investigation to include the ‘“‘alkali’’ soils of the salt-spots, one of which 
lies close to the Laboratory domain. As regards these, we are in posses- 
sion of the facts of distribution, but the causal relations involved are 
not wholly clear. How far the peculiar vegetation of the salt-spots and 
its arrangement, and the absence of most other plants from these areas 
are due, to use Dr. Livingston’s words, to the concentration of the soil 
solution, which is often too great to allow osmotic absorption by the 
roots of most plants, or how far the cause may lie in poor drainage and 
consequent defective aeration is still uncertain. It is suggested by the 
author, though no exerimental work in this direction is reported, that 
the distribution of plants is perhaps more often determined by avail- 
ability of oxygen than by that of water. This suggestion gains in force 
by what, as a conspicuous example, are shown to be the excellent drain- 
age conditions of the Larrea slope, and by what we have seen of the 
avoidance on the part of the creosote-bush of areas of defective drainage. 

Observations of aspect preference, which have been made both on 
the Laboratory domain and in other places at different elevations, have 
resulted in a large accumulation of facts which point to the conclusion 
that temperature is first of all a controlling factor in the determination 


126 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


of this preference, but do not make it clear, nor in any wise probable, 
that no other factors are involved. The behavior of the sahuaro and 
Encelva on southern exposures, their high temperature requirement, and 
their limitation in altitude and latitude, taken in connection with tem- 
perature records, all point to temperature as being of prime importance 
in determining the aspect preference of these plants. Similarly, the 
behavior of Lippia wright, evidently a plant of somewhat lower tem- 
perature requirements, finding its home on the north side of Tumamoc 
Hill at an altitude of 2,600 feet, but at 3,000 to 4,000 feet greater ele- 
vation requiring the warmth of southern exposures and occurring exclu- 
sively on them, leads to the same conclusion. It may well be, however, 
that many of the plants which on the Laboratory domain affect northern 
exposures find there congenial conditions not simply because they can 
sustain a lower temperature in winter or because the heat of summer 
is tempered, but because evaporation is less there than on southerly 
aspects. And even if at the outset, as seems plain from observations 
in the gulch near the Laboratory, temperature is proven to have been 
the controlling factor, changes taking place with the lapse of time, such 
as the accumulation of vegetable mold and the better retention of soil- 
moisture, accompanied by a relatively low rate of evaporation, may 
together produce a total of conditions which are semi-mesophytic in 
comparison with those of the bare rocks and rapidly drying soil of south- 
erly aspects as they lie exposed to the fierce glare of the sun. 

With this complicated set of relations it is apparently impossible at 
present to analyze aspect preferences causally further than to say that 
differences of temperature, corresponding with different temperature 
requirements on the part of the plants themselves, are without doubt of 
primary importance, while the degree to which other factors are involved 
is, aS yet, uncertain. Measurements of evaporation which are now being 
made may throw light on the problem. 

Thus, along all the several lines that have been followed, the evidence 
has accumulated rendering the conclusion a necessary one that percent- 
age of soil-water and temperature are two factors of the first importance 
in determining the local distribution of desert plants. There is also 
abundant evidence that aeration of the soil is still another highly impor- 
tant factor, but the data in regard to this are not sufficiently definite 
to admit of quantitative expression. The influence, in some cases con- 
spicuous, of high percentages of alkali and the complex of factors deter- 
mining evaporation must be admitted to the same category. 

That this conclusion is what might have been expected does not affect 
its value. It has all along been obvious that desert plants are governed 
by the same general laws in their relation to environmental conditions 
as are those of other regions, and the factors determining their distri- 
bution might a priori be assumed to be the same. ‘They exhibit, how- 


REVIEW AND DISCUSSION. 17 


ever, in their response to these factors differences of degree, if not of 
kind, and in the study of desert plants, which are exposed to wide ex- 
tremes of conditions in closely approximate areas, it is perhaps easier 
to estimate with an approach to certainty the relative influence of dif- 
ferent factors than it is in the case of plants of regions of abundant rain- 
fall. This has been attempted with a few plants of the Laboratory 
domain which exhibit conspicuous and definite habitat preferences. 


CONCURRENT ACTION OF DIFFERENT FACTORS. 


The case of Cereus giganteus is one of the best for the present purpose 
and will be taken first. This plant, a most striking feature of the land- 
scape wherever it occurs, has naturally attracted the attention of all 
observers, and its occurrence has been noted by so many that its western, 
northern, and eastern limits are perfectly well known. On the west it 
ranges to the close vicinity of the Gulf of California, and in two or three 
places passes into the State of California, just beyond the Colorado 
River. To the northward it is found as far as Bill Williams River and 
the Tonto Basin, and on the east to the San Pedro Valley. Only to 
the south, the distance it extends into Mexico is not yet recorded. 

The habits of the sahuaro, as they have been observed from what is 
probably the central part of its range to its northern limits, are such 
as to at once attract notice and, taken together, appear to admit of but 
one explanation. It is preeminently an inhabitant of rocky slopes and 
southern exposures. Its seeds germinate only at high temperatures. 
Its behavior at the limits of its northern growth, and at the limit of its 
growth in altitude, goes to show that it is a plant of high temperature 
requirements. On the north side of Tumamoc Hill it is dying out; at 
all events, a considerable number have died there within the past two 
or three years, and reproduction is nearly at a standstill. 

It has been shown that this species has a superficially placed root- 
system and that it utilizes with remarkable promptness the light rain- 
fall, which, over certain parts of its area of distribution, probably amounts 
often to no more than 2 or 3 inches a year. Its storage system is per- 
fectly adapted to the requirements of such a situation, being adjustable 
to either a heavy downpour, which may occur in the form of torrential 
summer showers, or to the more usual light rains which are separated by 
long intervals of drought. ‘Thus far, observations of temperature where 
the difference of density on south and north exposures in close proximity 
‘is well marked, go to show that the conditions for its growth are very 
nearly optimum where the minimum winter temperature of the soil at 
12 inches below the surface is 5° F. higher than on the nearby northern 
exposure, where the sahuaro is almost wanting and shows signs of dying 
out. 


128 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


Putting all these things together, we are in a position to formulate the 
following theoretical and at the same time highly probable statement 
concerning the distribution of the sahuaro and its causes. 

It is essentially a subtropical species and could not, under existing 
climatic conditions, or any that are supposed to have preceded those 
of the present time, have originated north of its present limits. If 
migrations have been made by it, the general course of its advance has 
been northward, rather than southward; but the fact must again be 
emphasized that on southern exposures in the vicinity of Tucson what 
appear to be essentially optimum conditions for its development pre- 
vail, and it is entirely conceivable that this plant has never, within all 
its history as a distinct species, undergone any extensive migrations 
whatever. 

As it stands to-day, its capacity for the utilization of light rains and 
of getting on with an exceedingly meager annual precipitation adapts 
the sahuaro, as regards the water relation, to a fairly wide range in the 
southwestern United States and northern Mexico, but this is greatly 
restricted by its inability to cope with low temperatures. We have, 
accordingly, in the giant cactus, a plant which, though extraordinary 
in the perfection of its adaptations to the arid regions in which it lives, 
is so limited by its lack of capacity to endure cold that, beyond certain 
lines, its progress to higher latitudes and altitudes is inevitably stopped, 
and locally, far within these limits, the successful occupation of northern 
exposures is impossible. ‘Thus bounds are set by its relation to one of 
_ these two factors, temperature and water-supply, to what, as far as the 
other is concerned, might be a far more extensive distribution than it 
exhibits to-day. There can be no doubt, moreover, that the limits of 
distribution of this species are defined by its physiological relation to 
these two physical factors; others, however important to plant life in 
general, may be omitted from consideration. ‘There is not a particle of 
evidence that light, for example, is to be reckoned as a factor limiting 
the distribution of the sahuaro. It would apparently find light enough 
for its needs hundreds of miles north and east of its present limits, and 
nowhere within its limits, as far as can be seen, does it suffer from too 
intense insolation. Here, however, observations of its deportment in 
the southern part of its range, which are not at hand, are greatly needed. 

If wide distribution is assumed to be an advantage to any given spe- 
cies of plant, the criticism of the giant cactus would be that while—with 
its superficially placed root-system and its exquisite mechanical structure, 
in short, with its essentially perfect construction as regards the water- 
relation—it is adapted to a much greater area of arid country than it 
has yet occupied, such occupation is rendered entirely impossible by its 
physiological relation to temperature. It will be understood that this 
applies to it in its northward extension. Whether the same principle 


REVIEW AND DISCUSSION. 129 


holds good as it approaches its southern limits we are not in possession 
of data to determine. 

From the studies thus far made we are also in possession of the key 
to a probable explanation of the association of this species with plants, 
such as the palo verde and others, of very different and considerably 
wider geographical range and of very different physiological requirements 
from those of the sahuaro, though at this place characteristic members 
of the same association with it. The superficial root-system of the 
sahuaro, as we have seen, gives it a decided advantage, as far as its 
capacity for utilizing light rains is concerned, but with their deeper root- 
systems the palo verde, the creosote-bush, and others are in a_ position 
to make use of water that has reached a lower level. ‘These plants, 
accordingly, which are so constantly associated here, experience a mu- 
tual advantage in the differences of their root-systems. 

To put it sharply, general similarity of climatic and dissimilarity of 
certain edaphic relations appears, in this case at least, to favor the asso- 
ciation of the plants in question. Not all of the roots of the palo verde 
and other species referred to extend deeply; some are superficial, and it 
may well be that in the layers of soil near the surface there may often 
be sharp competition between them and the sahuaro; but if so, a modus 
vivendi is found in the differences of their root-systems that have been 
pointed out, and it seems at least highly probable that this is an impor- 
tant means by which they are enabled to develop normally side by side. 


In marked contrast with the sahuaro in many respects, though fre- 
quently associated with it, is the creosote-bush (Larrea tridentata). ‘This 
plant presents no extraordinary structure or mechanism in any way 
comparable to that of the sahuaro, yet it is far more successful than 
the latter as a desert species. Both are well protected against excessive 
transpiration, a characteristic which they share with desert perennials 
in general, but in other respects it would be hard to find two plants more 
unlike. The distribution of the root-system of Larrea places it at once 
at a great advantage, making it capable of drawing its water-supply 
both from higher and lower levels. It can maintain itself with an exceed- 
ingly meager percentage of soil-water, as its occupation of the Larrea 
slope shows, but it luxuriates in an abundant supply, as is clearly seen 
by its vigorous growth in the wash and wherever plenty of water is given 
it. It bears successfully a far wider range of temperature than does 
the sahuaro, and with these physiological characteristics we find it, as 
would be expected, ranging far beyond the latter in both latitude and 
altitude, and in its local distribution affecting indifferently all exposures 
and all sorts of soils from the wash to the summit of Tumamoc Hill. To 
put all in a word, the tolerance of wide extremes, as regards both tem- 
perature and water-supply, on the part of this plant corresponds with 
its relatively wide geographical range and its local occupation of a great 


130 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


variety of habitats. Its success as a desert species is a striking illus- 
tration of the great advantage in adverse circumstances of physiological 
endurance over highly developed adaptations. In possession of this 
undefined quality, which, however, is capable of definite measurement, 
the creosote-bush ranges from below sea-level in the Salton Basin to 
nearly or quite 6,000 feet in altitude, and on a great variety of soils 
from central Texas to the foothills of the Coast Range, while its occur- 
rence (or a species so closely related as to be indistinguishable from it 
except by a systematist) in Chile and Argentina indicates its prolonged 
and successful fight with vicissitudes which we may now but faintly 
picture. All the evidence, however, points to the conclusion that while 
subject, like the sahuaro, to general physiological laws, its definite rela- 
tion to these two factors, temperature and water-supply, has determined 
the geographical range and the local distribution of the creosote-bush, 
and its wider capacity in regard to both of these, as compared with the 
sahuaro, has given it a foothold, both locally and at a distance, far beyond 
the power of the latter species to reach. 

There is, nevertheless, one point in which even the creosote-bush is 
at a disadvantage. A comparison of the maps (plates 14 and 18) shows, 
as already pointed out, that beyond the sharp line between the Tumamoc 
slope and the flood-plain of the Santa Cruz the creosote-bush does not 
go. It also shuns the salt-spots, and to the south of Tucson one may 
ride for miles through the bolson area which lies between the slopes of 
the Tucson Mountains and the Sierritas without noting a single speci- 
men. This avoidance of the flood-plain, seen in all the great river valleys 
of southern Arizona, of salt-spots, and of some bolson areas, seems 
to admit of but one explanation. The creosote-bush is preeminently a 
plant of well-drained ground. Whatever else it can endure, it is quite 
unable to cope with conditions that exist where there is defective aeration 
of the soil. Thus this most successful plant of the desert, the one which 
seems adapted to a wider range of conditions than any other, is brought 
to a full stop when it reaches the line between its own special habitat, 
that is, the one in which it is most numerously represented, and that 
of the mesquite and the salt-bushes. 


As we proceed to other plants it becomes plain that each species has 
its own physiological requirements, resulting in the introduction of very 
different elements, even into the same association of plants, and making 
the distribution of each a problem by itself. The case of Encelia jarinosa, 
a close companion of the sahuaro, and with the same limitations as to 
aspect preference, goes to show that two species, closely associated, may 
have essentially identical temperature requirements, and at the same 
time may differ as widely as possible in their relations to water-supply. 
Again, the sharp tension line between the domain of the creosote-bush 
and that of the mesquite appears to be due to the fact on the one hand 


REVIEW AND DISCUSSION. 131 


that the creosote-bush is more sensitive than the mesquite to deficiency 
of aeration, and-accordingly shuns the flood-plain, and on the other hand 
that the mesquite is less capable than the creosote-bush of living with 
an insufficient water-supply, and on this account makes no advances 
upon the Larrea slope. That they occur together in places on Tumamoc 
Hill indicates that in such spots satisfactory conditions both of aeration 
and of water-supply are present. 


LOCAL MOVEMENTS. 


The discussion thus far has related chiefly to statical rather than 
dynamical phenomena. Desert plants have been referred to their places 
and associations very much as if these were permanent as regards place 
and composition; but here as elsewhere, it is necessary, as a matter of 
fact, to conceive of movements, both of associations and of individual 
plants, as continually taking place, resulting in a change of place of the 
association and a change of its composition through the introduction or 
loss of constituent species. 

Such movements are ordinarily too slow to easily admit of direct obser- 
_ vation, but they are readily demonstrable by suitable means. Com- 
parative observation, supplemented by exact mapping of selected areas, 
makes it possible to gain a satisfactory view of the course of movement 
at the present time, and the maps, as a permanent record, afford a body 
of reliable data by means of which, in the future, movements taking 
_ place in intervening years may be recognized. A review of a few cases 
will serve to illustrate. 

The most carefully observed case thus far is that of alfilaria (Erodiwm 
cicutartum), the path of which from California into southern Arizona has 
been clearly traced. It is plain, as its history (p. 52) and a reference 
to the map (plate 20) indicates, that it reached the northern part of 
the Laboratory domain by the way of the Santa Cruz Valley, and has 
advanced in the course of a few years to points some rods beyond the 
Laboratory building. ‘The same is true of foxtail (Hordeum murinum), 
which seems to have had an essentially identical history, but to have 
entered the domain more recently, and, though well established, has 
spread less widely (plate 22). Both have followed wagon tracks to a 
great extent, as if brought by teamsters, and both are already occupying 
some areas so thickly that it is plain from simple inspection of adjacent 
areas that they take possession of ground from year to year that but 
for their presence would be occupied by other annuals. 

Very different are the cases of some of the perennials that have recently 
found a place on Tumamoc Hill. A single individual of Cercedrum 
torreyanum stands in the gulch near the Laboratory, where, in all prob- 
ability, the seed was brought not long ago from the wash, less than a 
quarter of a mile away. The ironwood (Olneya tesota), two or three 


132 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


individuals of which are found on the Tumamoc Hills, and about the 
same number of Semmondsta californica have their nearest representatives 
a mile or two to the west on the slope of the Tucson Mountains. ‘The 
single specimen of Yucca elata growing on the Laboratory domain is 
found in a shady place in the wash, several miles from its kind, which is 
well represented, at a higher altitude, on the slopes toward the Sierritas 
a number of miles away. 

These several perennials have made so little progress in the occupation 
of the ground that they can hardly be considered to have become con- 
stituent members of the associations in which they occur, and there is 
every reason to expect that they will later exhibit wide differences in 
the degree to which they become established in the places they have 
entered. The single individual of Yucca elata may possibly maintain or 
even reproduce itself to some extent in the sheltered nook it has found 
on the Laboratory domain, but it is at least quite as probable that it 
will die out and leave no trace behind. It is of interest to note that 
several species (Spheralcea coultert, Anagallis arvensis, and Erodium 
moschatum) have been found by Professor Thornber growing on the 
Laboratory domain, but only for a year or two, and subsequently have 
disappeared. The Cercidium, on the other hand, may unquestionably be 
regarded as a forerunner of its kind, which, as the processes of erosion 
and deposition finally make the gulch and wash continuous, may be 
expected to move up with the extension of the latter, just as it is doing in 
other places at the present time, and to become there, as it is now in the 
wash, one of the most numerous and characteristic species of its association. 
- Without multiplying examples, it may be gathered from these and 
similar cases that various species of both annuals and perennials have 
very recently entered the Laboratory domain; that some of these have 
rapidly multiplied and competed successfully with earlier occupants of 
the ground; that others are barely holding their places as single indi- 
viduals, making practically no progress, and with a highly problematical 
future before them; and that still others, now well settled in their respec- 
tive associations, are holding their places by accommodation rather than 
competition. Thus, arriving at different times and by various routes, 
members of the associations here represented have come to hold the 
most various relations with their associates. 

The terms just employed—competition and accommodation—are to be 
taken literally. The widely prevalent view according to which desert 
plants are to be considered in relation to their environment, rather than 
to each other, can only be accepted for those areas where distinctively 
desert conditions prevail; but where these conditions are modified, as 
they are on the flood-plain, in the wash, and on the hill, in short over 
a large part of the Laboratory domain, there, as we have seen, the different 
plants stand in close relation to each other, and their relations are of 


REVIEW AND DISCUSSION. loo 


the most varied character. The giant cactus, in its close association 
with the palo verde and other deep-rooted perennials, affords a good illus- 
tration of the mutual accommodation of two or more different plants 
which are capable of growing advantageously side by side by reason of 
differences in distribution of their root-systems, while other cases that 
have been cited (pp. 52-56) indicate something of the extent to which 
competition prevails between annuals and perennials, between different 
species of annuals, and between individuals of the same species. It 
would be difficult to imagine a case of greater complexity than is pre- 
sented by the mutual relations of the plants of the Laboratory domain, 
one in which generalization is less safe, or in which the necessity of study- 
ing every species by itself is more imperative. 

The associations here represented are in unstable equilibrium, not only 
as to their constituents, but also as to their place. The inevitable topo- 
graphical shifting observed in a region where erosion is both rapid and 
long-continued necessarily results in advance or retreat, or both, on the 
part of all the association groups. As the process of base-leveling pro- 
ceeds, the associations of the hill slowly give way before the upward 
movement of the creosote-bush association as it advances step by step, 
corresponding with the upward progress of the long slope to which it 
belongs. Most plainly, too, is seen the forward movement of the palo 
verde-catclaw association as the wash and its branches work their way 
slowly toward the mountains. Between the slope and the flood-plain is 
a line along which the relative positions of the associations on either side 
are determined by the preponderance of erosion or deposition, the creosote- 
bush characteristically belonging, as we have seen, to areas of erosion, the 
mesquite to those of deposition. 

Thus the movements of the associations as they are taking place at 
the present time are comparatively simple. The tendency of the topo- 
graphic changes that have been described is manifestly the building up 
of the long slopes at the expense of the hills and mountains, which now 
rise above them, and with this is necessarily associated the succession 
indicated in which the plants of the slopes become more and more domi- 
nant and the creosote-bush association ultimately represents for them 
the climax type. It is to be borne in mind, however, that in this region 
profound alteration of topographic features may at any time follow a 
new uplift or other phase of geologic activity, and accordingly we are in 
no position to predict changes which may greatly modify or retard the 
orderly succession which has been described and which is now actually 
going on in our sight. 

It is not possible, with present knowledge, to reproduce with certainty 
the history to this time of successive associations on Tumamoc Hill itself. 
The present stage exhibits phases that are more or less transitory, but 
it seems highly probable that the essential features of its vegetation have 


134 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


changed but little since it became established in its present pes pre- 
sumably as late as the Pleistocene period. 


GENERAL MOVEMENTS. 


During the whole of this study, which has been given chiefly to plants 
of the Laboratory domain, the wider problems of distribution have not 
been lost sight of, but thus far the available data have not been suffi- 
cient to justify any final statement of general conclusions. Several con- 
tributions by specialists, however, have presented much that is of value 
as a solid foundation for future work, and that points toward conclu- 
sions which may be adopted provisionally as a basis for more extended 
investigation. 

The geological history, by Professor Tolman, has an important bearing 
on the subject of plant distribution in the region in which the Desert 
Laboratory is situated, and although the relations brought out are largely 
of a general character, they are not on this account the less significant. 

(1) The relatively recent origin of Tumamoc Hill, and the general con- 
tinuity of its history since the various igneous outbreaks that have been 
described, render it probable that, in many cases at least, the elements 
of its flora came to it in possession of essentially the same characters 
as they exhibit at the present day. That such is the case on certain 
parts of the hill is rendered evident by what is seen where topographic 
changes that have been described are now taking place, and the new 
areas are being occupied by representatives of species already estab- 
lished in their neighborhood. 

It is safe to assume that the continuity of geological history has been 
paralleled by a like continuity of plant life, and that precisely as the 
slow progression of geological events is best understood by exact study 
of what is now taking place, so the distribution and movements of the 
plants of the Laboratory domain, through the period of existence of 
Tumamoc Hill, are best studied by obtaining a clear conception of what 
they are at the present day. ‘This principle, once admitted, indicates a 
safe and fruitful method of procedure. 

(2) The leading topographical features, which are a direct outcome of 
the forces and events described, are conspicuously correlated with clearly 
recognizable associations of plants. ‘The rock surface of the hill, the 
slopes below, and the alluvial plain of the Santa Cruz are so many diverse 
habitats, each with its distinctive associations of plants. The several topo- 
graphic areas maintain their relative positions, but are continually under- 
going changes due to erosion and deposition, and these are succeeded by 
movements of the plant associations, so gradually that the relations of the 
associations to each other are hardly disturbed. But with the topograph- 
ical features as described, there necessarily exist differences of gradient 
and aspect, and these latter especially are accompanied by wide differences 


~ 


REVIEW AND DISCUSSION. 135 


of physical conditions to which, in turn, correspond some of the most 
striking floristic differences observed on the Laboratory domain. ‘Thus 
the correlation of recent geological history and the movements of plants 
represented here becomes a matter subject to exact observation and record, 
as has been pointed out in previous sections. 

(3) The character of the rocks at this place does not appear, as a rule, 
to be intimately connected with differences of plant species, and this 
was to be expected from the general sameness of their chemical com- 
position. It is to be noted, however, that the rhyolite tuff, a conspic- 
uous formation of Tumamoc Hill, presenting a strong contrast to the 
dark-colored basalts, exhibits noticeable differences from the latter in 
its lichen flora, as has been pointed out by Professor Fink. The rapid 
formation of caliche described by Professor Tolman, in connection with 
what has been observed of its unfavorable character as a substratum 
for the growth of plants, may not improbably throw light on some facts 
of distribution not otherwise explained; but this has not yet been suf- 
ficiently investigated. 

(4) While emphasis has been laid upon the long-continued action of 
forces now in operation and upon the continuity of geological history, 
it is also necessary to take into account the fact that in the close vicinity 
of the Laboratory domain are areas the history of which extends farther 
into the past, and is more varied as regards climatic and other changes 
than is that of Tumamoc Hill. Whether this hill, however, was in a 
position to directly receive additions to its flora from the southward 
migration of the earlier Pleistocene, or received them later from neigh- 
boring areas is of comparatively little consequence. The evidence, chiefly 
botanical, that such migration took place is ample, and, by whatever 
way or means, ITumamoc Hill has received a share of the genera con- 
cerned in it. This part of the geological history of the region appears 
as yet to have been only partially made out, and until this has been 
accomplished no entirely definite account of its relations to the flora of 
the Laboratory domain can be looked for. 

It is seen from the foregoing that the critical study of a limited area 
from the geological standpoint, such as that presented by Professor 
Tolman, has proven to be of great value to the student of distribution, 
especially as regards general problems, and has also thrown light on 
various matters of local distribution. The present study furnishes abun- 
dant reason for enlarging the scope of the work and extending investi- 
gations of this kind more widely in the arid and semi-arid territory of 
the Southwest, where at this stage of botanical inquiry there is such 
pressing need of fuller knowledge of climatic conditions prevailing in 
later geological times. 


Through Prof. J. J. Thornber’s study of the local flora we are in pos- 
session of data bearing still more directly on the problem of the general 


136 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


movements of desert plants. From his analysis it is seen that within 
the area selected for special study there have been collected 449 species, 
belonging to 269 genera, included in 68 families of plants, and it is safe 
to assume that this list, as regards the flowering plants, approaches very 
closely to completeness. 

Of the 449 species no less than 264 are limited in their distribution to 
the southwestern United States and Mexico, in other words, at least 
59 per cent of our flora as it exists to-day is to be thought of as belong- 
' ing distinctively to the desert region of southwestern North America. 
It is of much interest to note, as pointed out by Professor Thornber, 
that of the species constituting the remaining 41 per cent a large pro- 
portion occur on the flood-plain, which may be considered the least 
distinctively desert part of the area studied. 

The classification of genera on the basis of their geographical distri- 
bution indicates that the flora of the Laboratory domain and the adjacent 
valley is composed of the following elements: 

(1) Genera belonging exclusively to the southwestern United States and 
Mexico, or occurring very exceptionally beyond those limits. Such are 
Fouquerta, Harpagonella, Keberlinia, Olneya, Riddellia, Simmondsia, and 
others, 39 in all. 

(2) North American genera, comparatively few in number, whose 
‘‘centers of dispersal,’ judged by criteria given by Adams (1902, p. 122) 
are in the northwestern and western United States. Eviogonum is a 
good example of these. 

(3) Genera well represented in high northern latitudes of both hemi- 
spheres, but occurring southward along the Alleghenies and the moun- 
tains of western North America. These are represented by Anemone, 
Delphintum, and others. They are far more numerous on the Santa 
Catalina and other neighboring mountains, where they constitute a large 
and characteristic element of the flora. 

(4) Tropical or subtropical American genera, including those that 
occur chiefly in the tropics, from Mexico southward, but exceptionally 
or not at all northward of Arizona. Examples of these are Condalia, 
Janusia, Martynia, Pectis, and others. 

(5) Genera ranging from the southwestern United States to extratrop- 
ical South America, e. g., Amsinckia, Argemone, Bowlesia, Cryptanthe, 
Encehia, Krameria, Larrea, and Trixis. ‘These constitute a characteristic 
element of the desert floras of Chile and Argentina. 

(6) Genera of both eastern and western hemispheres, many being 
widely distributed in both, such as Abutzlon, Acacia, Atriplex, Clematis, 
Ephedra, Hibiscus, Lycium, Sambucus. 

The natural interpretation of these data harmonizes with the well- 
known views of Gray, Engler (1882), and others. We may assume with 
regard to the geographical origin and movements of the plants here 


REVIEW AND: DISCUSSION. 137 


represented that a large contingent of Sonoran species had its origin in 
this immediate region, some of them possibly on the very ground of the 
Laboratory domain. Other species, relatively few in number, belong to 
genera which exhibit their greatest morphological differentiation and 
most numerous representation in the northwestern and western United 
States. The recent history of alfilaria shows with the greatest clearness 
one of the routes by which certain plants indigenous to California, or 
previously settled there, have reached us. 

Other plants have a history interwoven with that of the geology of 
western America, and accordingly it can only be stated in general terms. 
The outlines as sketched by Bray (1900) indicate that changes of ele- 
vation of the continental axis, together with climatic changes: dating 
from late Tertiary times, are the occasion of the wide and more or less 
discontinuous range of closely related species, which now grow on the 
plains of the southwestern United States and those of Chile and Argen- 
tina. At a relatively early day, possibly not later than the Pliocene, 
there seems to have been a continuous highway, since closed, from south- 
ern Mexico, along the Isthmus of Panama, southward to Argentina and 
Patagonia, by which the creosote-bush, various cacti, yuccas, certain 
composites and members of other groups represented on the Laboratory 
domain at the present day, made their southward journeys. In the 
northern hemisphere the later advance of the great ice-sheet in the Pleis- 
tocene drove southward many plants, some of which, as Anemone and 
Delphinvum, are found to-day on Tumamoc Hill, but in general they 
have made the mountains their pathway and also their home, and their 
minor movements can not well be traced. 

Of representatives of genera belonging to both hemispheres, including 
miscellaneous introduced species, it can only be said that they have 
entered the Laboratory domain by the most various routes and agencies. 
Each of these presents a separate history of its own, which in a few cases 
is well-known, while in others it remains undeciphered. 

The region in which the Desert Laboratory is situated has not only 
been the recipient of plants of other regions, but it has contributed its 
quota to other floras. Examples of this, in addition to those already 
given, are some of the cacti which have nearly or quite gained the British 
boundary in their northward movements. 







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CHAPTER VII. 
SUMMARY. 


The work thus far completed, with the most important results, may 
be summarized as follows: 

(1) Within the limits of the Desert Laboratory domain and that part 
of the Santa Cruz Valley immediately adjacent, chosen as a representa- 
tive area for the study of desert plants in southern Arizona, 12 plant 
associations have been recognized and defined. These fall naturally into 
A groups corresponding with the main topographical features of the area 
under observation. ‘The local distribution of certain species of these 
associations has been carefully mapped with special reference to topo- 
graphical and soil relations, by this means confirming conclusions based 
on previous observation. 

(2) Observations within this limited area, supplemented by compara- 
tive studies in the Gila Valley and elsewhere, have led to the conclusion 
that soil properties and aspect are of paramount importance in deter- 
mining the local distribution of desert plants. With regard to the former, 
evidence has been gained that soil-water exercises a controlling influence, 
but that, with certain species at least, aeration and percentage of alkali 
salts are also efficient factors. 

(3) These conclusions have been confirmed by investigations of the 
soils of the Laboratory domain conducted by Dr. B. E. Livingston. The 
correspondence between per cent of soil-moisture and the distribution of 
both the plant associations and their constituent species is especially 
striking and convincing. 

(4) The importance of aspect in determining distribution on closely 
adjacent areas has been shown by many different observations, but 
especially by floristic comparison of opposite sides of the gulch near the 
Laboratory, where, on equivalent areas, 2.5 times as many species are 
found on the northerly as on the southerly exposure, and the difference 
in number of individual plants is still more striking. 

Analysis of aspect preference, in connection with its observation at 
different altitudes and continued records of temperature, leads to the 
conclusion that it is correlated first of all with range of temperature, 
though other factors, in certain cases at least, are involved. It appears 
clear that whatever else is involved the lower temperatures of winter 
on northern exposures at this place have interfered with their occupa- 
tion, to any considerable extent, by the sahuaro and Fncelza, while the 
extreme heat of southern exposures in summer has, at this altitude, 
prevented their occupation by various species, of which Lippra wraghtir 
is a notable example. The behavior of this plant at different altitudes 

139 


140 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


affords confirmatory evidence of the fundamental importance of tem- 
perature in the determination of aspect preference. é 

(5) Light has been thrown on the causal relations involved in habitat 
preferences by the study of inherited habits and structures of certain 
species. Some at least, and possibly a large proportion of the plants 
which have become established within the Laboratory domain, have 
entered it with correspondingly great differences of habit and power of 
accommodation to conditions which exist here. These differences are con- 
spicuously manifested in choice of habitat, especially as regards aspect 
and soil preferences, and also in their relation to other members of the 
same association. fEncelia is a genus of subtropical species, chiefly, and 
its representative on the Laboratory domain finds most congenial con- 
ditions on southerly exposures, just as Delphinium and other plants of 
cooler and moister regions find their places here on northern slopes. The 
characteristics of the root-system of Cereus giganteus which have been 
worked out by Dr. W. A. Cannon, taken in connection with its arrange- 
ments for water storage and high temperature requirement, satisfactorily 
explain its limitations in both local and general distribution, pp. 59-66. 

Peculiarities of structure and habits, correlated with water-supply on 
the one hand and temperature relations on the other, are exhibited by a 
large percentage of plants of the Laboratory domain. These structures 
and habits have determined, and are still determining, the continued 
existence of the various species in places where they are now growing. 

(6) It is none the less certain, however, that there are here examples 
of plants in which the capacity for individual adjustment to changes 
of condition is a highly potent factor in the determination of local dis- 
tribution. The creosote-bush presents a case in point. It can not be 
doubted that to its extraordinary power of adjustment this plant owes 
its capacity of retaining its place on the long slopes, where, under the 
extremely adverse conditions there prevailing, it forms a conspicuous 
belt of vegetation, from which other perennial plants are largely absent, 
and at the same time flourishes in the low ground of the wash and grows 
luxuriantly along irrigating ditches, where it has an abundant water- 
supply. The same plant, together with the ocotillo and some others, 
shows its capacity for individual adjustment in its perfect indifference 
to change of aspect, to which so many species of Tumamoc Hill are 
exceedingly sensitive. The ready growth of Suweda moquini in the worst 
salt-spots, and at the same time along irrigating ditches conveying fresh 
water, is still another illustration. 

Fundamentally these cases of adjustment are the same as those of 
adaptation just cited. In the one case, as in the other, inherited peculi- 
arities determine the limits of choice. It is true, however, that a species 
without prominent structural adaptations may, by its inherited capacity 
of ready physiological adjustment, be a far more successful element of 


SUMMARY. 141 


desert vegetation than some more highly specialized forms. ‘The remark- 
able adaptations of the sahuaro are in strong contrast with the simple 
structures of the creosote-bush, but the latter ranges far more widely 
than the former, and stubbornly endures vicissitudes to which the sahuaro, 
even in the areas of its best development, inevitably succumbs. 

(7) It has become evident that the relations of desert plants to each 
other are not less important than their relations to their physical envi- 
ronment. The commonly received view that desert plants are engaged 
in a struggle with their environment, and not with each other, requires 
material modification in view of what has been observed to take place 
on Tumamoc Hill, where vigorous competition between different species 
and different individuals of the same species goes on from year to year. 
There is also a mutual accommodation, in certain cases, by which plants 
with root-systems reaching different levels are enabled to live advan- 
tageously in close proximity. It is plain that no general statement of 
the relation of desert plants to each other can be formulated at present. 
Each species requires investigation in its relation to its immediate asso- 
ciates, and when, as must happen, both competition and accommodation 
are involved the complicated nature of the problem is manifest. 

(8) Passing to matters pertaining to general movements of desert plants, 
it may be said that the agencies and structures operative in the dissem- 
ination of seeds and propagative bodies of plants on the domain of the 
Desert Laboratory present no special features, so far recognizable, and 
no unusual interpretation of their action is necessary to account for the 
presence of the species found here. 

The plants which have been brought by various efficient agencies are 
of widely different geographical origin. ‘The analysis of the flora, par- 
ticularly as regards the genera, indicates for some of them northern 
Mexico as their center of dispersal, for others tropical or subtropical 
America, for another contingent the northwestern United States, and 
for another the high northern regions of either hemisphere, while still 
others, including miscellaneous introduced species, have come from vari- 
ous parts of the world by the most diverse routes, some of which have 
been satisfactorily traced. 

(9) It is noteworthy that between 400 and 500 species of plants, of 
such diverse geographical origins, should have been able to establish 
themselves within the narrow limits of the Tumamoc Hills and the adja- 
cent valley and accommodate themselves to the somewhat trying cli- 
- matic conditions prevailing here, since the effectiveness of general climatic 
factors in limiting the range of species is well known. Important in 
this connection are the observations of Professor Thornber on the relation 
between the annual distribution of rainfall and vegetation in southern 
Arizona, according to which it appears that grasses are favored by the 
preponderance of summer rainfall to the east and south of Tucson, in 
contrast to preponderating winter rainfall to the west. 


142 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 


The experimental work of Dr. Livingston on evaporation has gone far 
to provide an efficient means of testing the ecological equivalence of 
habitats, and this work goes to show the value and practicality of maps 
of distribution based on the principle of such equivalency, which would 
serve, as nothing else could, as a rational basis for various horticultural 
undertakings, that at present are in an almost wholly empirical stage. 

(10) The general continuity of geological history since the Tertiary 
indicates a relatively long period within which plants of the Laboratory 
domain have one by one, or at any rate by no mass movements, become 
established in their places. There is reason to believe that throughout 
this period the processes now going on before our eyes have been in 
progress. ‘The present flora, therefore, may be assumed to be merely 
the final stage, thus far, of just such a series of events as are now observ- 
able. Invasions have taken place, competition has ensued, certain spe- 
cies have become established as prominent constituents of the various 
associations, while others have died out or taken subordinate places. 
Species now growing side by side have reached Tumamoc Hill from 
widely separated regions and at intervals of possibly thousands of years 
apart. The small area within its limits has received representatives of 
genera that have shared in the great migrations south and north along 
the Cordilleras, but through the time that has elasped since these greater 
movements it has also received, by entirely ordinary means, the plants 
that have come and are still coming to it. The general history is not 
essentially affected by the fact that portions of Tumamoc Hill are of 
quite recent origin. These parts give evidence of having received their 
~ flora in the main from the immediate neighborhood, although certain 
genera have come from points at some little distance, without settling 
in the less favorable intervening territory. 

If, then, as the evidence seems to indicate, the same geological agencies 
have been operating in the same way from the Tertiary down to the 
present, and generally arid conditions have prevailed, it is safe to say 
that during that long period the domain of the Desert Laboratory has 
suffered no greater, if as great, extremes of climate as prevail to-day in 
places no farther apart than Tucson and the Santa Catalina Mountains. 
It seems likely that to-day, within areas only a few miles apart, we may 
see floras more diverse in character than the late Tertiary and present- 
day floras of the Laboratory domain. As a corollary it may be assumed 
that, neglecting minor divisions, there have existed in this region, at least 
from the late Tertiary, two widely different floras, that of the plains and 
that of the mountains, the former of essentially desert character, the latter 
of mesophytic species, and that during this period, without losing at any 
time their distinctive features, they have, by precisely the same agencies 
as are operative to-day, received accessions, lost or passed on waning 
species, and otherwise suffered gradual modification. 


LITERATURE CITED. 


Avams, C. C. 
1902. Southeastern United States as a center of geographical distribution of 
flora and fauna. Biol. Bull., 3: 121, 122. 
BaILEY, F. M. 
1902. Handbook of birds of the western United States. LxxXIV-LXxxII. 
BAILEY, V. 
1905. U.S. Dept. Agric., Biol. Sur., N. A. Fauna, 25, Biological Survey of Texas. 
BLUMER, J. C. 
1908. Some observations on Arizona fungi. ‘The Plant World, 11: 14-17. 
1908. Distributional features of some southwestern shrubs. The Plant World, 
bie oit 7-7 23. 
Bray, W. L. 
1900. Plant geography of North America. ‘The relations of the North American 
flora to that of South America. Science, n.s., 12: 709-716. 
1904. Forest resources of Texas. U.S. Dept. Agric., Bur. Forestry, Bull. 47: 
34, 57: 
Briaces, L. J. 
1899. Electrical instruments for determining the moisture, temperature, and 
soluble salt contents of soils. U.S. Dept. Agric., Bur. Soils, Bull. 15. 
Briccs, L. J., and McLane#, J. W. 
1907. ‘The moisture equivalents of soils. U.S. Dept. Agric., Bur. Soils, Bull. 45. 
CAMERON, F. K., and GALLAGHER, F. E. 
1908. Moisture content and physical condition of soils. U. S$. Dept. Agric., 
Bur. Soils, Bull. 50. 
CANNON, W. A. 
1904. Observations on the germination of Phoradendron villosum and P. califor- 
nicum. Bull. Tor. Bot. Club, 31: 435-443. 
1905. On the transpiration of Fouqueria splendens. Bull. Tor. Bot. Club, 32: 
All. 
1905. Onthewater-conducting systemsof some desert shrubs. Bot. Gaz., 39: 407 
1908. On the electric resistance of solutions of salt plants and solutions of alkali 
soils. The Plant World, 11: 11. 
ENGLER, A. 
1882. Versuch einer Entwicklungs-geschichte der Pflanzenwelt, 2. 
FERNALD, M. L. 
1907. ‘The soil preferences of certain alpine and subalpine plants. Rhodora, 
9: 149-193. 
FINK, B. 


1899. Contributions to a knowledge of the lichens of Minnesota. V. Minn. 
Bot. Stud., 2: 286-288. 
1904. A lichen society of sandstone riprap. Bot. Gaz., 38: 269-279. 
JENNINGS, O. E. 
1908. A note on the ecological formations of Pittsburg and vicinity. Science, 
n. s., 27: 828-830. 
JUMELLE, H. 
1892. Recherches physiologiques sur les lichens. Rev. gén. bot., 4 : 115. 
KEARNEY, T. H., and Harter, L. L. 
1907. ‘The comparative tolerance of various plants for the salts common in alkali 
soils. U.S. Dept. Agric., Bur. Pl. Ind., Bull. 113. 
LapHaM, M. H., and NEw, N. P. 
1904. Soil survey of the Solomonsville area, Arizona. U.S. Dept. Agric., Bur. Soils. 
LIVINGSTON, B. E. 
1906. ‘The relation of desert plants to soil moisture and evaporation. Publication 
50, Carnegie Institution of Washington. 
1907. Evaporation and plant development. ‘The Plant World, 10: 269-278. 
1908. Evaporation and plant habitats. The Plant World, 11: 1-9, 106-112. 
ELovp, HL. E- 
1904. <A botanical laboratory in the desert. Pop. Sci. Mo., 66: 329-342. 
1906. ‘The artificial induction of leaf formation in the ocotillo, ‘The Plant World, 
9: 56-59. 
143 


144 LITERATURE CITED. 


MacDouca., D. T. 
1908. The botanical features of North American deserts. Publication 99, Car- 
negie Institution of Washington. 
MeErRRIAM, C. H. 
1898. Ife zones and crop zones of the United States. U.S. Dept. Agric., Div. 
Biol. Sur., Bull. 10. 
NEw York STaTE Acric. Exp. STa., 1886, Ann. Rep., 4: 176-179. 
RuTHVEN, A. G. 
1907. A collection of reptiles and amphibians from southern New Mexico and 
Arizona. Bull. Am. Mus. Nat. Hist., 23: 483-604. 
SPALDING, E. S. 
1905. Mechanical adjustment of the sahuaro (Cereus giganteus) to varying quan- 
tities of stored water. Bull. Tor. Bot. Club, 32: 57-68. 
SPALDING, V. M. 
1906. Biological relations of desert shrubs. II. Absorption of water by leaves. 
Bot. Gaz., 41: 262-282. , 
1906. Absorption of atmospheric moisture by desert shrubs. Bull. Tor. Bot. 
Club, 33: 367-375. 
THORNBER, J. J. 
1906. Alfilaria (Evodvum cicutarrwm) as a forage plant in Arizona. Ariz. Agric. 
Exp; pta.. Bulls 52: 
TOWER, W. L. 
1906. An investigation of evolution in Chrysomelid beetles of the genus Leptino- 
tarsa. Publication 48, Carnegie Institution of Washington. 
TRANSEAU, E. N. 
1905. Forest centers of eastern America. Am. Nat., 39: 875-880. 
1905. Climatic centers and centers of plant distribution. Mich. Acad. Sci. Ann. 
RED 4 3515: 
1908. The relation of plant societies to evaporation. Bot. Gaz., 45: 217-231. 
WHEELER, W. M. 
1907. ‘The polymorphism of ants, with an account of some singular abnormalities 
due to parasitism. Bull. Am. Mus. Nat. Hist., 23 : 1-93. 
1907. The fungus-growing ants of North America. Bull. Am. Mus. Nat. Hist., 
23: 669-807. 
ZUKAL, H. 
1895. Morphologische und biologische Untersuchungen iiber die Flechten. Sitz- 
ungsbericht. Kaiserl. Akad. Wien, 14: 1308. 





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